SkLS accepts nullptr for pointer args, handles alpha accurately, has new GM

src/core/SkLightingShader.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkLightingShader_DEFINED
 #define SkLightingShader_DEFINED
 
 #include "SkLights.h"
 #include "SkShader.h"
 
 class SkBitmap;
 class SkMatrix;
 class SkNormalSource;
 
 class SK_API SkLightingShader {
 public:
     /** Returns a shader that lights the shape, colored by the diffuseShader, using the
         normals from normalSource, with the set of lights provided.
 
         It returns a shader with a reference count of 1.
         The caller should decrement the shader's reference count when done with the shader.
         It is an error for count to be < 2.
-        @param  diffuseShader     the shader that provides the colors
-        @param  normalSource      the source for the shape's normals
+        @param  diffuseShader     the shader that provides the colors. If nullptr, uses the paint's
+                                  color.
+        @param  normalSource      the source for the shape's normals. If nullptr, assumes straight
+                                  up normals (<0,0,1>).
         @param  lights            the lights applied to the normals
 
         The lighting equation is currently:
             result = LightColor * DiffuseColor * (Normal * LightDir) + AmbientColor
 
     */
     static sk_sp<SkShader> Make(sk_sp<SkShader> diffuseShader, sk_sp<SkNormalSource> normalSource,
                                 sk_sp<SkLights> lights);
 
     SK_DECLARE_FLATTENABLE_REGISTRAR_GROUP()
 };
 
 #endif