func (gc *GraphicContext) ClearRect(x1, y1, x2, y2 int) {
	mask := gc.newMask(x1, y1, x2-x1, y2-y1)

	newGroup := &Group{
		Groups: gc.svg.Groups,
		Mask:   "url(#" + mask.Id + ")",
	}

	// replace groups with new masked group
	gc.svg.Groups = []*Group{newGroup}
}
func (gc *GraphicContext) ClearRect(x1, y1, x2, y2 int) {
	mask := gc.newMask(x1, y1, x2-x1, y2-y1)

	newGroup := &Group{
		Groups: gc.svg.Groups,
		Mask:   "url(#" + mask.Id + ")",
	}

	// replace groups with new masked group
	gc.svg.Groups = []*Group{newGroup}
}
// ClearRect fills the specified rectangle with a default transparent color