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

src/core/SkShader.cpp

Index: src/core/SkShader.cpp
diff --git a/src/core/SkShader.cpp b/src/core/SkShader.cpp
index 305cf06695376d83848c8a47ed2f3ba251e04375..abd0e9a0ca57bcd520bae0009798504b521dad82 100644
--- a/src/core/SkShader.cpp
+++ b/src/core/SkShader.cpp
@@ -79,6 +79,18 @@ bool SkShader::computeTotalInverse(const ContextRec& rec, SkMatrix* totalInverse
     return m->invert(totalInverse);
 }
 
+bool SkShader::asLuminanceColor(SkColor* colorPtr) const {
+    SkColor storage;
+    if (NULL == colorPtr) {
+        colorPtr = &storage;
+    }
+    if (this->onAsLuminanceColor(colorPtr)) {
+        *colorPtr = SkColorSetA(*colorPtr, 0xFF);   // we only return opaque
+        return true;
+    }
+    return false;
+}
+
 SkShader::Context* SkShader::createContext(const ContextRec& rec, void* storage) const {
     if (!this->computeTotalInverse(rec, NULL)) {
         return NULL;