todo
参考：mob-sakai/SoftMaskForUGUI: Enhance Unity UI (uGUI) with advanced soft-masking features to create more visually appealing effects!
【SHADER系列】（四）UGUI 渐变/平滑遮罩 SoftMask
todo
参考：mob-sakai/SoftMaskForUGUI: Enhance Unity UI (uGUI) with advanced soft-masking features to create more visually appealing effects!