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

src/core/SkNormalSource.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 SkNormalSource_DEFINED
 #define SkNormalSource_DEFINED
 
@@ skipping 47 matching lines @@
         is retrieved by:
             N.x = R-127;
             N.y = G-127;
             N.z = B-127;
             N.normalize();
         The +Z axis is thus encoded in RGB as (127, 127, 255) while the -Z axis is
         (127, 127, 0).
     */
     static sk_sp<SkNormalSource> MakeFromNormalMap(sk_sp<SkShader> map, const SkMatrix& ctm);
 
+    /** Returns a normal source that provides straight-up normals only (0, 0, 1).
+    */
+    static sk_sp<SkNormalSource> MakeFlat();
+
     SK_DEFINE_FLATTENABLE_TYPE(SkNormalSource)
     SK_DECLARE_FLATTENABLE_REGISTRAR_GROUP()
 };
 
 #endif