Refine snapshot invalidation and modal resize behavior

Canvas.cpp

@@ -72,7 +72,7 @@ void Canvas::draw()
 		// 如果位置或尺寸变了，或没有有效缓存，则重新抓取
 		if (!saveBkImage || saveBkX != this->x - margin || saveBkY != this->y - margin || saveWidth != this->width + margin * 2 || saveHeight != this->height + margin * 2)
 		{
-			discardBackground();
+			invalidateBackgroundSnapshot();
 			saveBackground(this->x - margin, this->y - margin, this->width + margin * 2, this->height + margin * 2);
 		}
 	}
@@ -231,7 +231,7 @@ void Canvas::setIsVisible(bool visible)
 		control->setIsVisible(visible);
 	}
 	if (!visible)
-		this->updateBackground();
+		discardBackground();
 }
 
 void Canvas::setDirty(bool dirty)