Always fill Box background

This resolves rendering issues with TextViews which have their background
color set to ColorDefault.

box.go

@@ -255,13 +255,11 @@ func (b *Box) Draw(screen tcell.Screen) {
 
 	// Fill background.
 	background := def.Background(b.backgroundColor)
-	if b.backgroundColor != tcell.ColorDefault {
 	for y := b.y; y < b.y+b.height; y++ {
 		for x := b.x; x < b.x+b.width; x++ {
 			screen.SetContent(x, y, ' ', nil, background)
 		}
 	}
-	}
 
 	// Draw border.
 	if b.border && b.width >= 2 && b.height >= 2 {