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 }