add ability to get FBO ID to Surface

src/image/SkSurface_Gpu.cpp

Index: src/image/SkSurface_Gpu.cpp
diff --git a/src/image/SkSurface_Gpu.cpp b/src/image/SkSurface_Gpu.cpp
index 71bed3a9b9083a70bbc1afc9603f61df66ee0ea5..75f9471f3e1d0c2964d336e8f86650dc6dc31f7f 100644
--- a/src/image/SkSurface_Gpu.cpp
+++ b/src/image/SkSurface_Gpu.cpp
@@ -26,20 +26,36 @@ SkSurface_Gpu::~SkSurface_Gpu() {
     fDevice->unref();
 }
 
-GrBackendObject SkSurface_Gpu::onGetTextureHandle(TextureHandleAccess access) {
-    GrRenderTarget* rt = fDevice->accessRenderTarget();
+static GrRenderTarget* prepare_rt_for_external_access(SkSurface_Gpu* surface,
+                                                      SkSurface::BackendHandleAccess access) {
+    GrRenderTarget* rt = surface->getDevice()->accessRenderTarget();
     switch (access) {
-        case kFlushRead_TextureHandleAccess:
+        case SkSurface::kFlushRead_BackendHandleAccess:
             rt->prepareForExternalRead();   // todo: rename to prepareForExternalAccess()
             break;
-        case kFlushWrite_TextureHandleAccess:
-        case kDiscardWrite_TextureHandleAccess:
+        case SkSurface::kFlushWrite_BackendHandleAccess:
+        case SkSurface::kDiscardWrite_BackendHandleAccess:
             // for now we don't special-case on Discard, but we may in the future.
-            this->notifyContentWillChange(kRetain_ContentChangeMode);
+            surface->notifyContentWillChange(SkSurface::kRetain_ContentChangeMode);
             rt->flushWrites();
             break;
     }
-    return rt->asTexture()->getTextureHandle();
+    return rt;
+}
+
+GrBackendObject SkSurface_Gpu::onGetTextureHandle(BackendHandleAccess access) {
+    GrRenderTarget* rt = prepare_rt_for_external_access(this, access);
+    if (rt->asTexture()) {
+        return rt->asTexture()->getTextureHandle();
+    } else {
+        return NULL;
+    }
+}
+
+bool SkSurface_Gpu::onGetRenderTargetHandle(GrBackendObject* obj, BackendHandleAccess access) {
+    GrRenderTarget* rt = prepare_rt_for_external_access(this, access);
+    *obj = rt->getRenderTargetHandle();
+    return true;
 }
 
 SkCanvas* SkSurface_Gpu::onNewCanvas() {