sixel: Export $FZF_PREVIEW_TOP to the preview command (#2544)

So that it can determine if it should subtract 1 from $FZF_PREVIEW_LINES
to avoid scrolling issue of Sixel image that touches the bottom of the
screen.

src/tui/dummy.go

@@ -41,6 +41,7 @@ func (r *FullscreenRenderer) Close()                             {}
 func (r *FullscreenRenderer) Size() TermSize                     { return TermSize{} }
 
 func (r *FullscreenRenderer) GetChar() Event { return Event{} }
+func (r *FullscreenRenderer) Top() int       { return 0 }
 func (r *FullscreenRenderer) MaxX() int      { return 0 }
 func (r *FullscreenRenderer) MaxY() int      { return 0 }
 

src/tui/light.go

@@ -720,6 +720,10 @@ func (r *LightRenderer) Close() {
 	r.restoreTerminal()
 }
 
+func (r *LightRenderer) Top() int {
+	return r.yoffset
+}
+
 func (r *LightRenderer) MaxX() int {
 	return r.width
 }

src/tui/tcell.go

@@ -172,6 +172,10 @@ func (r *FullscreenRenderer) Init() {
 	initTheme(r.theme, r.defaultTheme(), r.forceBlack)
 }
 
+func (r *FullscreenRenderer) Top() int {
+	return 0
+}
+
 func (r *FullscreenRenderer) MaxX() int {
 	ncols, _ := _screen.Size()
 	return int(ncols)

src/tui/tui.go

@@ -494,6 +494,7 @@ type Renderer interface {
 
 	GetChar() Event
 
+	Top() int
 	MaxX() int
 	MaxY() int