Bug 90443

The key point of this patch is to change how BitmapImage uses ImageSource. Originally, BitmapImage retrieves the frame data from ImageSource by calling the following methods: - NativeImagePtr createFrameAtIndex(size_t); - float frameDurationAtIndex(size_t); - bool frameHasAlphaAtIndex(size_t); - bool frameIsCompleteAtIndex(size_t); - ImageOrientation orientationAtIndex(size_t) const; Upon each method call, ImageSource synchronously decodes the image and returns the requested information. Because all the methods above are inherently synchronous, it is not easy to make them parallel. To implement parallel image decoders, these methods must be asynchronous. So I merged all these methods into a single method, requestFrameAtIndex(size_t), which requests all the frame data at once. Note that this method has no return value. Instead, BitmapImage now implements ImageSourceObserver and passes itself to ImageSource constructor. Once image decoding is finished or cached image is available, ImageSource notifies BitmapImage via ImageSourceObserver::didReceiveImageFrameData(size_t, const ImageFrameData&). ImageFrameData contains all the frame data including frame, duration, isAlpha, isComplete and orientation.