Add a set of image filters

With this you can do variants of this:

```
{{ $img := resources.Get "images/misc/3-jenny.jpg" }}
{{ $img := $img.Resize "300x" }}
{{ $g1 := $img.Filter images.Grayscale }}
{{ $g2 := $img | images.Filter (images.Saturate 30) (images.GaussianBlur 3) }}
```

Fixes #6255

89 files changed, 791 insertions, 139 deletions

diff --git a/go.mod b/go.mod
index 662da9360..cff19abc7 100644
--- a/go.mod
+++ b/go.mod
@@ -12,7 +12,7 @@ require (
 	github.com/bep/debounce v1.2.0
 	github.com/bep/gitmap v1.1.0
 	github.com/bep/go-tocss v0.6.0
-	github.com/disintegration/imaging v1.6.0
+	github.com/disintegration/gift v1.2.1
 	github.com/dustin/go-humanize v1.0.0
 	github.com/eknkc/amber v0.0.0-20171010120322-cdade1c07385
 	github.com/fortytw2/leaktest v1.3.0
diff --git a/go.sum b/go.sum
index e5ae159b6..43275e38a 100644
--- a/go.sum
+++ b/go.sum
@@ -88,8 +88,8 @@ github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSs
 github.com/dgrijalva/jwt-go v3.2.0+incompatible/go.mod h1:E3ru+11k8xSBh+hMPgOLZmtrrCbhqsmaPHjLKYnJCaQ=
 github.com/dgryski/go-sip13 v0.0.0-20181026042036-e10d5fee7954/go.mod h1:vAd38F8PWV+bWy6jNmig1y/TA+kYO4g3RSRF0IAv0no=
 github.com/dimchansky/utfbom v1.1.0/go.mod h1:rO41eb7gLfo8SF1jd9F8HplJm1Fewwi4mQvIirEdv+8=
-github.com/disintegration/imaging v1.6.0 h1:nVPXRUUQ36Z7MNf0O77UzgnOb1mkMMor7lmJMJXc/mA=
-github.com/disintegration/imaging v1.6.0/go.mod h1:xuIt+sRxDFrHS0drzXUlCJthkJ8k7lkkUojDSR247MQ=
+github.com/disintegration/gift v1.2.1 h1:Y005a1X4Z7Uc+0gLpSAsKhWi4qLtsdEcMIbbdvdZ6pc=
+github.com/disintegration/gift v1.2.1/go.mod h1:Jh2i7f7Q2BM7Ezno3PhfezbR1xpUg9dUg3/RlKGr4HI=
 github.com/dlclark/regexp2 v1.1.6 h1:CqB4MjHw0MFCDj+PHHjiESmHX+N7t0tJzKvC6M97BRg=
 github.com/dlclark/regexp2 v1.1.6/go.mod h1:2pZnwuY/m+8K6iRw6wQdMtk+rH5tNGR1i55kozfMjCc=
 github.com/dustin/go-humanize v1.0.0 h1:VSnTsYCnlFHaM2/igO1h6X3HA71jcobQuxemgkq4zYo=
@@ -338,8 +338,6 @@ golang.org/x/crypto v0.0.0-20181203042331-505ab145d0a9/go.mod h1:6SG95UA2DQfeDnf
 golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
 golang.org/x/crypto v0.0.0-20190422183909-d864b10871cd/go.mod h1:yigFU9vqHzYiE8UmvKecakEJjdnWj3jj499lnFckfCI=
 golang.org/x/exp v0.0.0-20190121172915-509febef88a4/go.mod h1:CJ0aWSM057203Lf6IL+f9T1iT9GByDxfZKAQTCR3kQA=
-golang.org/x/image v0.0.0-20180708004352-c73c2afc3b81 h1:00VmoueYNlNz/aHIilyyQz/MHSqGoWJzpFv/HW8xpzI=
-golang.org/x/image v0.0.0-20180708004352-c73c2afc3b81/go.mod h1:ux5Hcp/YLpHSI86hEcLt0YII63i6oz57MZXIpbrjZUs=
 golang.org/x/image v0.0.0-20190523035834-f03afa92d3ff h1:+2zgJKVDVAz/BWSsuniCmU1kLCjL88Z8/kv39xCI9NQ=
 golang.org/x/image v0.0.0-20190523035834-f03afa92d3ff/go.mod h1:kZ7UVZpmo3dzQBMxlp+ypCbDeSB+sBbTgSJuh5dn5js=
 golang.org/x/lint v0.0.0-20181026193005-c67002cb31c3/go.mod h1:UVdnD1Gm6xHRNCYTkRU2/jEulfH38KcIWyp/GAMgvoE=
diff --git a/resources/image.go b/resources/image.go
index e1a816942..7113284f7 100644
--- a/resources/image.go
+++ b/resources/image.go
@@ -16,18 +16,19 @@ package resources
 import (
 	"fmt"
 	"image"
-	"image/color"
 	"image/draw"
 	_ "image/gif"
 	_ "image/png"
 	"os"
 	"strings"
 
+	"github.com/gohugoio/hugo/resources/internal"
+
 	"github.com/gohugoio/hugo/resources/resource"
 
 	_errors "github.com/pkg/errors"
 
-	"github.com/disintegration/imaging"
+	"github.com/disintegration/gift"
 	"github.com/gohugoio/hugo/helpers"
 	"github.com/gohugoio/hugo/resources/images"
 
@@ -82,16 +83,26 @@ func (i *imageResource) cloneWithUpdates(u *transformationUpdate) (baseResource,
 // filter and returns the transformed image. If one of width or height is 0, the image aspect
 // ratio is preserved.
 func (i *imageResource) Resize(spec string) (resource.Image, error) {
-	return i.doWithImageConfig("resize", spec, func(src image.Image, conf images.ImageConfig) (image.Image, error) {
-		return i.Proc.Resize(src, conf)
+	conf, err := i.decodeImageConfig("resize", spec)
+	if err != nil {
+		return nil, err
+	}
+
+	return i.doWithImageConfig(conf, func(src image.Image) (image.Image, error) {
+		return i.Proc.ApplyFiltersFromConfig(src, conf)
 	})
 }
 
 // Fit scales down the image using the specified resample filter to fit the specified
 // maximum width and height.
 func (i *imageResource) Fit(spec string) (resource.Image, error) {
-	return i.doWithImageConfig("fit", spec, func(src image.Image, conf images.ImageConfig) (image.Image, error) {
-		return i.Proc.Fit(src, conf)
+	conf, err := i.decodeImageConfig("fit", spec)
+	if err != nil {
+		return nil, err
+	}
+
+	return i.doWithImageConfig(conf, func(src image.Image) (image.Image, error) {
+		return i.Proc.ApplyFiltersFromConfig(src, conf)
 	})
 }
 
@@ -99,8 +110,22 @@ func (i *imageResource) Fit(spec string) (resource.Image, error) {
 // crops the resized image to the specified dimensions using the given anchor point.
 // Space delimited config: 200x300 TopLeft
 func (i *imageResource) Fill(spec string) (resource.Image, error) {
-	return i.doWithImageConfig("fill", spec, func(src image.Image, conf images.ImageConfig) (image.Image, error) {
-		return i.Proc.Fill(src, conf)
+	conf, err := i.decodeImageConfig("fill", spec)
+	if err != nil {
+		return nil, err
+	}
+
+	return i.doWithImageConfig(conf, func(src image.Image) (image.Image, error) {
+		return i.Proc.ApplyFiltersFromConfig(src, conf)
+	})
+}
+
+func (i *imageResource) Filter(filters ...gift.Filter) (resource.Image, error) {
+	conf := i.Proc.GetDefaultImageConfig("filter")
+	conf.Key = internal.HashString(filters)
+
+	return i.doWithImageConfig(conf, func(src image.Image) (image.Image, error) {
+		return i.Proc.Filter(src, filters...)
 	})
 }
 
@@ -118,19 +143,14 @@ const imageProcWorkers = 1
 
 var imageProcSem = make(chan bool, imageProcWorkers)
 
-func (i *imageResource) doWithImageConfig(action, spec string, f func(src image.Image, conf images.ImageConfig) (image.Image, error)) (resource.Image, error) {
-	conf, err := i.decodeImageConfig(action, spec)
-	if err != nil {
-		return nil, err
-	}
-
+func (i *imageResource) doWithImageConfig(conf images.ImageConfig, f func(src image.Image) (image.Image, error)) (resource.Image, error) {
 	return i.getSpec().imageCache.getOrCreate(i, conf, func() (*imageResource, image.Image, error) {
 		imageProcSem <- true
 		defer func() {
 			<-imageProcSem
 		}()
 
-		errOp := action
+		errOp := conf.Action
 		errPath := i.getSourceFilename()
 
 		src, err := i.decodeSource()
@@ -138,17 +158,12 @@ func (i *imageResource) doWithImageConfig(action, spec string, f func(src image.
 			return nil, nil, &os.PathError{Op: errOp, Path: errPath, Err: err}
 		}
 
-		if conf.Rotate != 0 {
-			// Rotate it before any scaling to get the dimensions correct.
-			src = imaging.Rotate(src, float64(conf.Rotate), color.Transparent)
-		}
-
-		converted, err := f(src, conf)
+		converted, err := f(src)
 		if err != nil {
 			return nil, nil, &os.PathError{Op: errOp, Path: errPath, Err: err}
 		}
 
-		if i.Format == imaging.PNG {
+		if i.Format == images.PNG {
 			// Apply the colour palette from the source
 			if paletted, ok := src.(*image.Paletted); ok {
 				tmp := image.NewPaletted(converted.Bounds(), paletted.Palette)
@@ -222,7 +237,7 @@ func (i *imageResource) re