extend SkShader to report a luminance-color to be used for gamma correction

src/core/SkPaint.cpp

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkPaint.h"
 #include "SkAnnotation.h"
 #include "SkAutoKern.h"
@@ skipping 1479 matching lines @@
     SkPaint::Hinting h = paint.getHinting();
     if (paint.isLinearText()) {
         h = SkPaint::kNo_Hinting;
     }
     return h;
 }
 
 // return true if the paint is just a single color (i.e. not a shader). If its
 // a shader, then we can't compute a const luminance for it :(
 static bool justAColor(const SkPaint& paint, SkColor* color) {
-    if (paint.getShader()) {
+    SkColor c = paint.getColor();
+
+    SkShader* shader = paint.getShader();
+    if (shader && !shader->asLuminanceColor(&c)) {
         return false;
     }
-    SkColor c = paint.getColor();
     if (paint.getColorFilter()) {
         c = paint.getColorFilter()->filterColor(c);
     }
     if (color) {
         *color = c;
     }
     return true;
 }
 
 static SkColor computeLuminanceColor(const SkPaint& paint) {
@@ skipping 1162 matching lines @@
     F_UNREF(Looper,      readDrawLooper);
     F_UNREF(ImageFilter, readImageFilter);
     F(Typeface,    readTypeface);
 #undef F
 #undef F_UNREF
     if (dirty & kAnnotation_DirtyBit) {
         paint->setAnnotation(SkAnnotation::Create(buffer))->unref();
     }
     SkASSERT(dirty == paint->fDirtyBits);
 }