Fix scrollability of the preview window in certain cases

Fix #2683

This commit fixes the cases where fzf incorrectly determines the
scrollability of the preview window when `--preview-window-wrap` is set.

Wrapping of the preview content happens during the rendering phase, so
it's currently not possible to know how many lines are actually needed
to display the content beforehand. So `preview-bottom` still may not
move to the very bottom with wrapping enabled.

src/terminal.go

@@ -1417,6 +1417,7 @@ func (t *Terminal) renderPreviewText(height int, lines []string, lineNo int, unc
 		line = strings.TrimSuffix(line, "\n")
 		if lineNo >= height || t.pwindow.Y() == height-1 && t.pwindow.X() > 0 {
 			t.previewed.filled = true
+			t.previewer.scrollable = true
 			break
 		} else if lineNo >= 0 {
 			var fillRet tui.FillReturn