Changes

(Note: the description below applies to the "layers mode" integration with WebRender, which is deprecated now. We are in the process of transitioning to the "layers-free mode", where Gecko display items are directly converted to WebRender commands via their CreateWebRenderCommands functions. Some of the content below still applies though; in particular we still create a WebRenderLayerManager, which manages the conversion from Gecko display items to WebRender display items.) If you are familiar with the way Gecko normally works, the Quantum Render changes should be relatively straightforward. Instead of using a ClientLayerManager, we create a [https://hg.mozilla.org/projects/graphics/file/tip/gfx/layers/wr/WebRenderLayerManager.cpp WebRenderLayerManager] instance. This However, this "layer manager creates its own layer instances" doesn't actually manage layers, because WebRender doesn't use layers. Instead, it walks the Gecko display list and when those layers have their Render() calls the <tt>BuildWebRenderCommands</tt> function called, they on each display item to generate WebRender-specific commandsdisplay items. These commands are This display list is sent over the [https://hg.mozilla.org/projects/graphics/file/tip/gfx/layers/ipc/PWebRenderBridge.ipdl PWebRenderBridge] IPDL channel, which is conceptually similar to PLayerTransaction in Gecko. The commands are received in [https://hg.mozilla.org/projects/graphics/file/tip/gfx/layers/wr/WebRenderBridgeParent.cpp WebRenderBridgeParent] which interprets the messages, and talks to the core WebRender library using the API in [https://hg.mozilla.org/projects/graphics/file/tip/gfx/webrender_bindings/webrender_ffi.h webrender_ffi.h] (via various wrapper abstractions in <tt>gfx/webrender_bindings/</tt>). The API is implemented in the [https://hg.mozilla.org/projects/graphics/file/tip/gfx/webrender_bindings/src/bindings.rs webrender_bindings] crate, which is written in Rust.