(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

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)
 	}