Changes

It is important to understand the separation between layers and compositables. Compositables handle all the logic around texture transfer, while layers define the shape of the layer tree. a Compositable is created independently from a layer and attached to it on both sides. While layers transactions can only originate forme from the content thread, this separation make makes it possible for us to have separate compositable transactions between any thread and the compositor thread. We use the ImageBridge IPDL protocol to that end. The Idea of ImageBridge, is to create a Compositable that is manipulated on the ImageBridgeThread, and that can transfer/synchronize textures without using the content thread at all. This is very useful for smooth Video compositing: Video video frames are decoded and passed into the ImageBridge with without ever touching the content thread, which could be busy processing reflows or heavy javascript workloads.