Optimizations and more documentation of SkShadowShader

src/core/SkShadowShader.h

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 
 #ifndef SkShadowShader_DEFINED
 #define SkShadowShader_DEFINED
 
 #ifdef SK_EXPERIMENTAL_SHADOWING
 
 class SkLights;
 class SkShader;
 
 class SK_API SkShadowShader {
 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,
                                 sk_sp<SkLights> lights,
                                 int diffuseWidth, int diffuseHeight,
                                 const SkShadowParams& params);
 
     // The shadow shader supports any number of ambient lights, but only
     // 4 non-ambient lights (currently just refers to directional lights).
     static constexpr int kMaxNonAmbientLights = 4;
 
     SK_DECLARE_FLATTENABLE_REGISTRAR_GROUP()
 };
 
 #endif
 #endif