Issue #2357 - Paused WebM videos w/alpha are 100% transparent if HA is disabled.

Mozilla found a bug in their initial implementation, causing paused WebM videos with alpha to become totally transparent if hardware acceleration is disabled. Straight port of the Firefox 54 fix.

Ref: BZ 1332952

gfx/layers/ipc/SharedRGBImage.cpp

@@ -106,7 +106,32 @@ SharedRGBImage::GetTextureClient(KnowsCompositor* aForwarder)
 already_AddRefed<gfx::SourceSurface>
 SharedRGBImage::GetAsSourceSurface()
 {
-  return nullptr;
+  NS_ASSERTION(NS_IsMainThread(), "Must be main thread");
+
+  if (mSourceSurface) {
+    RefPtr<gfx::SourceSurface> surface(mSourceSurface);
+    return surface.forget();
+  }
+
+  RefPtr<gfx::SourceSurface> surface;
+  {
+    // We are 'borrowing' the DrawTarget and retaining a permanent reference to
+    // the underlying data (via the surface). It is in this instance since we
+    // know that the TextureClient is always wrapping a BufferTextureData and
+    // therefore it won't go away underneath us.
+    BufferTextureData* decoded_buffer =
+      mTextureClient->GetInternalData()->AsBufferTextureData();
+    RefPtr<gfx::DrawTarget> drawTarget = decoded_buffer->BorrowDrawTarget();
+
+    if (!drawTarget) {
+      return nullptr;
+    }
+
+    surface = drawTarget->Snapshot();
+  }
+
+  mSourceSurface = surface;
+  return surface.forget();
 }
 
 } // namespace layers

gfx/layers/ipc/SharedRGBImage.h

@@ -51,6 +51,7 @@ private:
   gfx::IntSize mSize;
   RefPtr<ImageClient> mCompositable;
   RefPtr<TextureClient> mTextureClient;
+  nsCountedRef<nsMainThreadSourceSurfaceRef> mSourceSurface;
 };
 
 } // namespace layers