Clear background even if background color is not set

This is needed when fzf is started from inside a program (e.g. Vim)
and it uses a different background color than the terminal.

- https://github.com/junegunn/fzf.vim/issues/325
- https://github.com/junegunn/fzf.vim/issues/300

src/terminal.go

@@ -600,7 +600,7 @@ func (t *Terminal) resizeWindows() {
 			width,
 			height, tui.BorderNone)
 	}
-	if !t.tui.IsOptimized() && t.theme != nil && t.theme.HasBg() {
+	if !t.tui.IsOptimized() {
 		for i := 0; i < t.window.Height(); i++ {
 			t.window.MoveAndClear(i, 0)
 		}