Optimizations and more documentation of SkShadowShader

src/core/SkShadowShader.h

Index: src/core/SkShadowShader.h
diff --git a/src/core/SkShadowShader.h b/src/core/SkShadowShader.h
index 3263950a93910691b6fd985256a7d0b81d8107f0..ea05ccae89d63cb718a9c4a5912bb3ed85b833b3 100644
--- a/src/core/SkShadowShader.h
+++ b/src/core/SkShadowShader.h
@@ -19,6 +19,9 @@ public:
     /** This shader combines the diffuse color in 'diffuseShader' with the shadows
      *  determined by the 'povDepthShader' and the shadow maps stored in each of the
      *  lights in 'lights'
+     *
+     *  Please note that the shadow shader is required to be in Stage0, otherwise
+     *  the texture coords will be wrong within the shader.
      */
     static sk_sp<SkShader> Make(sk_sp<SkShader> povDepthShader,
                                 sk_sp<SkShader> diffuseShader,