refine fastShadow (to best match non-fastShadow effects)

Runtime/Resources/shaders/UIWidgets_canvas_shadowBox.shader

     //originally from http://madebyevan.com/shaders/fast-rounded-rectangle-shadows/
     Properties
     {
-        _SrcBlend("_SrcBlend", Int) = 1 // One
-        _DstBlend("_DstBlend", Int) = 10 // OneMinusSrcAlpha
         _StencilComp("_StencilComp", Float) = 8 // - Equal, 8 - Always 
     }
     SubShader
-        Blend [_SrcBlend] [_DstBlend]
+        Blend SrcAlpha OneMinusSrcAlpha
         
         Stencil {
             Ref 128

Runtime/Resources/shaders/UIWidgets_canvas_shadowRBox.shader

     //originally from http://madebyevan.com/shaders/fast-rounded-rectangle-shadows/
     Properties
     {
-        _SrcBlend("_SrcBlend", Int) = 1 // One
-        _DstBlend("_DstBlend", Int) = 10 // OneMinusSrcAlpha
         _StencilComp("_StencilComp", Float) = 8 // - Equal, 8 - Always 
     }
     SubShader
-        Blend [_SrcBlend] [_DstBlend]
+        Blend SrcAlpha OneMinusSrcAlpha
         
         Stencil {
             Ref 128

Runtime/Resources/shaders/computebuffer/UIWidgets_canvas_shadowBox_cb.shader

     //originally from http://madebyevan.com/shaders/fast-rounded-rectangle-shadows/
     Properties
     {
-        _SrcBlend("_SrcBlend", Int) = 1 // One
-        _DstBlend("_DstBlend", Int) = 10 // OneMinusSrcAlpha
         _StencilComp("_StencilComp", Float) = 8 // - Equal, 8 - Always 
     }
     SubShader
-        Blend [_SrcBlend] [_DstBlend]
+        Blend SrcAlpha OneMinusSrcAlpha
         
         Stencil {
             Ref 128

Runtime/Resources/shaders/computebuffer/UIWidgets_canvas_shadowRBox_cb.shader

     //originally from http://madebyevan.com/shaders/fast-rounded-rectangle-shadows/
     Properties
     {
-        _SrcBlend("_SrcBlend", Int) = 1 // One
-        _DstBlend("_DstBlend", Int) = 10 // OneMinusSrcAlpha
         _StencilComp("_StencilComp", Float) = 8 // - Equal, 8 - Always 
     }
     SubShader
-        Blend [_SrcBlend] [_DstBlend]
+        Blend SrcAlpha OneMinusSrcAlpha
         
         Stencil {
             Ref 128

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

             }
 
             var bound =  path.getBounds();
-            var sigma = state.scale * paint.maskFilter.Value.sigma;
+            var sigma = state.scale * paint.maskFilter.Value.sigma / 3f;
             
             var vertices = ObjectPool<uiList<Vector3>>.alloc();
             vertices.SetCapacity(4);
             ObjectPool<uiMeshMesh>.release(meshBounds);
             ObjectPool<uiMeshMesh>.release(blurMesh);
             var mesh = uiMeshMesh.create(state.matrix, vertices, _triangles);
-            layer.draws.Add(CanvasShader.fastShadow(layer, mesh, sigma, path.isRect, path.isCircle, path.rRectCorner, new Vector4(bound.left, bound.top, bound.right, bound.bottom), paint.color));
+            var shadowColor = paint.color.withAlpha(128);
+            layer.draws.Add(CanvasShader.fastShadow(layer, mesh, sigma, path.isRect, path.isCircle, path.rRectCorner, new Vector4(bound.left, bound.top, bound.right, bound.bottom), shadowColor));
         }
 
     }