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

src/image/SkSurface.cpp

Index: src/image/SkSurface.cpp
diff --git a/src/image/SkSurface.cpp b/src/image/SkSurface.cpp
index ecb80427576de2ae2b99bf8235662d8820f65315..edc6ef5fa0e6dd225a0b22b453605f3587c52789 100644
--- a/src/image/SkSurface.cpp
+++ b/src/image/SkSurface.cpp
@@ -43,11 +43,7 @@ void SkSurface_Base::onDraw(SkCanvas* canvas, SkScalar x, SkScalar y,
 void SkSurface_Base::aboutToDraw(ContentChangeMode mode) {
     this->dirtyGenerationID();
 
-    if (NULL != fCachedCanvas) {
-        SkASSERT(fCachedCanvas->getSurfaceBase() == this || \
-                 NULL == fCachedCanvas->getSurfaceBase());
-        fCachedCanvas->setSurfaceBase(NULL);
-    }
+    SkASSERT(!fCachedCanvas || fCachedCanvas->getSurfaceBase() == this);
 
     if (NULL != fCachedImage) {
         // the surface may need to fork its backend, if its sharing it with
@@ -61,12 +57,13 @@ void SkSurface_Base::aboutToDraw(ContentChangeMode mode) {
         // that the next request will get our new contents.
         fCachedImage->unref();
         fCachedImage = NULL;
+    } else if (kDiscard_ContentChangeMode == mode) {
+        this->onDiscard();
     }
 }
 
 uint32_t SkSurface_Base::newGenerationID() {
-    this->installIntoCanvasForDirtyNotification();
-
+    SkASSERT(!fCachedCanvas || fCachedCanvas->getSurfaceBase() == this);
     static int32_t gID;
     return sk_atomic_inc(&gID) + 1;
 }