Bug 1938053 - Accelerate canvas drawImage of a WebGL context. r=aosmond

Accelerated Canvas2D and WebGL both live within the GPU process and run within
the same thread. We want to avoid any kind of readbacks from the GPU process
to the content process when doing a drawImage of a WebGL context to an AC2D
canvas.

To achieve this, we pause the AC2D recording translation with an AwaitTranslationSync
event identified by a sync-id. Then we send a request over IPDL to snapshot the
WebGL context while this pause is ongoing via a SnapshotExternalCanvas IPDL message,
which uses the sync-id to identify the snapshot safely in a table of such external
snapshots and force translation to resume. Finally, we send a ResolveExternalSnapshot
event within the recording stream to lookup the snapshot based on the sync-id and
assign it an alias that can be used within the recording stream playback for drawImage.

The sync-id mechanism acts as a sequenced fence so that multiple SnapshotExternalCanvas
requests can be encountered simultaneously from IPDL without confusing the recording
playback.

Differential Revision: https://phabricator.services.mozilla.com/D243399

dom/canvas/ClientWebGLContext.cpp

@@ -1142,6 +1142,11 @@ already_AddRefed<gfx::SourceSurface> ClientWebGLContext::GetSurfaceSnapshot(
   return ret.forget();
 }
 
+mozilla::ipc::IProtocol* ClientWebGLContext::SupportsSnapshotExternalCanvas()
+    const {
+  return GetChild();
+}
+
 RefPtr<gfx::SourceSurface> ClientWebGLContext::GetFrontBufferSnapshot(
     const bool requireAlphaPremult) {
   const FuncScope funcScope(*this, "<GetSurfaceSnapshot>");