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

src/core/SkLightingShader.h

Index: src/core/SkLightingShader.h
diff --git a/src/core/SkLightingShader.h b/src/core/SkLightingShader.h
index bb642615a56d512d57769206f853d610066e8685..41e3ca239440cb9e92e491958e258c3a7a249e5f 100644
--- a/src/core/SkLightingShader.h
+++ b/src/core/SkLightingShader.h
@@ -23,8 +23,10 @@ public:
         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: