Address some SkLightingShader TODOs

This CL:
  switches the light colors to be 3 scalars (SkColor3f)
  adds some dox

Review URL: https://codereview.chromium.org/1265983003
diff --git a/include/effects/SkPoint3.h b/include/effects/SkPoint3.h
index f9b727b..c086931 100644
--- a/include/effects/SkPoint3.h
+++ b/include/effects/SkPoint3.h
@@ -109,5 +109,6 @@
 };
 
 typedef SkPoint3 SkVector3;
+typedef SkPoint3 SkColor3f;
 
 #endif