Bug 1777426 - Add gfx.canvas.accelerated.async-present for toggling async present with Canvas2D independent of WebGL. r=aosmond

Differential Revision: https://phabricator.services.mozilla.com/D150881

dom/canvas/DrawTargetWebgl.cpp

@@ -2926,6 +2926,9 @@ Maybe<layers::SurfaceDescriptor> DrawTargetWebgl::GetFrontBuffer() {
       // Copy and swizzle the WebGL framebuffer to the swap chain front buffer.
       webgl::SwapChainOptions options;
       options.bgra = true;
+      // Allow async present to be toggled on for accelerated Canvas2D independent
+      // of WebGL via pref.
+      options.forceAsyncPresent = StaticPrefs::gfx_canvas_accelerated_async_present();
       mSharedContext->mWebgl->CopyToSwapChain(mFramebuffer, options);
     }
   }

modules/libpref/init/StaticPrefList.yaml

@@ -5250,6 +5250,11 @@
 #endif
   mirror: always
 
+- name: gfx.canvas.accelerated.async-present
+  type: RelaxedAtomicBool
+  value: true
+  mirror: always
+
 - name: gfx.canvas.accelerated.cache-items
   type: RelaxedAtomicUint32
   value: 2048