(Experimental) Improve Sixel graphics support (#2544)

Progress:

* Sixel image can now be displayed with other text, and is scrollable
* If an image can't be displayed entirely due to the scroll offset, fzf
  will render a wireframe to indicate that an image should be displayed
* Renamed $FZF_PREVIEW_{WIDTH,HEIGHT} to $FZF_PREVIEW_PIXEL_{WIDTH,HEIGHT}
  for clarity
* Added bin/fzf-preview.sh script to demonstrate how to display an image
  using Kitty or Sixel protocol

An example:

  ls *.jpg | fzf --preview='seq $((FZF_PREVIEW_LINES*9/10)); fzf-preview.sh {}; seq 100'

A known issue:

* If you reduce the size of the preview window, the image may extend
  beyond the preview window

5 files changed, 16 insertions, 15 deletions

diff --git a/src/tui/dummy.go b/src/tui/dummy.go
index d893a747..13c2aeec 100644
--- a/src/tui/dummy.go
+++ b/src/tui/dummy.go
@@ -38,9 +38,7 @@ func (r *FullscreenRenderer) Clear()                             {}
 func (r *FullscreenRenderer) NeedScrollbarRedraw() bool          { return false }
 func (r *FullscreenRenderer) Refresh()                           {}
 func (r *FullscreenRenderer) Close()                             {}
-func (r *FullscreenRenderer) Size() (termSize, error) {
-	return termSize{}, nil
-}
+func (r *FullscreenRenderer) Size() TermSize                     { return TermSize{} }
 
 func (r *FullscreenRenderer) GetChar() Event { return Event{} }
 func (r *FullscreenRenderer) MaxX() int      { return 0 }
diff --git a/src/tui/light.go b/src/tui/light.go
index 6b7eaaf4..e5080d1d 100644
--- a/src/tui/light.go
+++ b/src/tui/light.go
@@ -1092,7 +1092,9 @@ func (w *LightWindow) CFill(fg Color, bg Color, attr Attr, text string) FillRetu
 }
 
 func (w *LightWindow) FinishFill() {
-	w.MoveAndClear(w.posy, w.posx)
+	if w.posy < w.height {
+		w.MoveAndClear(w.posy, w.posx)
+	}
 	for y := w.posy + 1; y < w.height; y++ {
 		w.MoveAndClear(y, 0)
 	}
diff --git a/src/tui/light_unix.go b/src/tui/light_unix.go
index 4ca847b4..46188869 100644
--- a/src/tui/light_unix.go
+++ b/src/tui/light_unix.go
@@ -110,10 +110,10 @@ func (r *LightRenderer) getch(nonblock bool) (int, bool) {
 	return int(b[0]), true
 }
 
-func (r *LightRenderer) Size() (termSize, error) {
+func (r *LightRenderer) Size() TermSize {
 	ws, err := unix.IoctlGetWinsize(int(r.ttyin.Fd()), unix.TIOCGWINSZ)
 	if err != nil {
-		return termSize{}, err
+		return TermSize{}
 	}
-	return termSize{int(ws.Row), int(ws.Col), int(ws.Xpixel), int(ws.Ypixel)}, nil
+	return TermSize{int(ws.Row), int(ws.Col), int(ws.Xpixel), int(ws.Ypixel)}
 }
diff --git a/src/tui/tcell.go b/src/tui/tcell.go
index 54feaf16..cd723e32 100644
--- a/src/tui/tcell.go
+++ b/src/tui/tcell.go
@@ -203,9 +203,10 @@ func (r *FullscreenRenderer) Refresh() {
 	// noop
 }
 
-func (r *FullscreenRenderer) Size() (termSize, error) {
+// TODO: Pixel width and height not implemented
+func (r *FullscreenRenderer) Size() TermSize {
 	cols, lines := _screen.Size()
-	return termSize{lines, cols, 0, 0}, error("Not implemented")
+	return TermSize{lines, cols, 0, 0}
 }
 
 func (r *FullscreenRenderer) GetChar() Event {
diff --git a/src/tui/tui.go b/src/tui/tui.go
index 69ae8a1a..2ebb5e72 100644
--- a/src/tui/tui.go
+++ b/src/tui/tui.go
@@ -473,11 +473,11 @@ func MakeTransparentBorder() BorderStyle {
 		bottomRight: ' '}
 }
 
-type termSize struct {
-	Lines   int
-	Columns int
-	Width   int
-	Height  int
+type TermSize struct {
+	Lines    int
+	Columns  int
+	PxWidth  int
+	PxHeight int
 }
 
 type Renderer interface {
@@ -497,7 +497,7 @@ type Renderer interface {
 	MaxX() int
 	MaxY() int
 
-	Size() (termSize, error)
+	Size() TermSize
 
 	NewWindow(top int, left int, width int, height int, preview bool, borderStyle BorderStyle) Window
 }