Merge branch 'optmize_shadow' of github.com:UnityTech/UIWidgets into dev

# Conflicts:
#	Runtime/ui/painting/path.cs
#	Runtime/ui/renderer/cmdbufferCanvas/rendering/canvas_impl.cs
#	Runtime/ui/renderer/cmdbufferCanvas/rendering/canvas_shader.cs

Author: zhuxingwei

Runtime/ui/renderer/cmdbufferCanvas/rendering/canvas_shader.cs

@@ -176,6 +176,7 @@ static CanvasShader() {
             var filterShader = GetShader("UIWidgets/canvas_filter");
             var shadowBoxShader = GetShader("UIWidgets/ShadowBox");
             var shadowRBoxShader = GetShader("UIWidgets/ShadowRBox");
+            var strokeAlphaShader = GetShader("UIWidgets/canvas_strokeAlpha");
 
             _convexFillMat = new MaterialByBlendModeStencilComp(convexFillShader);
             _fill0Mat = new MaterialByStencilComp(fill0Shader);