Add discard API to SkCanvas, plumb it to glDiscardFramebuffer()

src/gpu/GrRenderTarget.cpp

Index: src/gpu/GrRenderTarget.cpp
diff --git a/src/gpu/GrRenderTarget.cpp b/src/gpu/GrRenderTarget.cpp
index f18df2cfa6d71b9b2c665438a9dbf243e65c42a0..729aa0d469a086f5d1a3b1ce8b301cb3c4acc7f5 100644
--- a/src/gpu/GrRenderTarget.cpp
+++ b/src/gpu/GrRenderTarget.cpp
@@ -54,6 +54,15 @@ void GrRenderTarget::resolve() {
     context->resolveRenderTarget(this);
 }
 
+void GrRenderTarget::discard() {
+    // go through context so that all necessary flushing occurs
+    GrContext* context = this->getContext();
+    if (NULL == context) {
+        return;
+    }
+    context->discardRenderTarget(this);
+}
+
 size_t GrRenderTarget::sizeInBytes() const {
     size_t colorBits;
     if (kUnknown_GrPixelConfig == fDesc.fConfig) {