(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/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
 }