resource: Use MD5 to identify image files

But only a set of byte chunks spread around in the image file to calculate the fingerprint, which is much faster than reading the whole file:

```bash
BenchmarkMD5FromFileFast/full=false-4         	  300000	      4356 ns/op	     240 B/op	       5 allocs/op
BenchmarkMD5FromFileFast/full=true-4          	   30000	     42899 ns/op	   32944 B/op	       5 allocs/op
```

Fixes #4186

1 files changed, 12 insertions, 0 deletions

diff --git a/resource/resource.go b/resource/resource.go
index 2c934d031..19392f3d3 100644
--- a/resource/resource.go
+++ b/resource/resource.go
@@ -153,7 +153,19 @@ func (r *Spec) newResource(
 	gr := r.newGenericResource(linker, fi, absPublishDir, absSourceFilename, filepath.ToSlash(relTargetFilename), mimeType)
 
 	if mimeType == "image" {
+		f, err := r.Fs.Source.Open(absSourceFilename)
+		if err != nil {
+			return nil, err
+		}
+		defer f.Close()
+
+		hash, err := helpers.MD5FromFileFast(f)
+		if err != nil {
+			return nil, err
+		}
+
 		return &Image{
+			hash:            hash,
 			imaging:         r.imaging,
 			genericResource: gr}, nil
 	}