Bug 1284324 - Implement Canvas Layer mirrors r=bas

- When a canvas layer is set to mirror, it copies the
  texture from the canvas rather than changing the
  texture factory with Morph().
- This is useful when a canvas content will be sent to
  multiple devices simultaneously, such as a VR HMD
  and a 2d monitor mirror.
- This is used by the WebVR 1.0 API, in Bug 1250244

MozReview-Commit-ID: JfMSockO2uz

gfx/layers/client/CanvasClient.cpp

@@ -411,6 +411,11 @@ CanvasClientSharedSurface::UpdateRenderer(gfx::IntSize aSize, Renderer& aRendere
       gfxCriticalError() << "Invalid canvas front buffer";
       return;
     }
+  } else if (layer && layer->mIsMirror) {
+    mShSurfClient = CloneSurface(gl->Screen()->Front()->Surf(), layer->mFactory.get());
+    if (!mShSurfClient) {
+      return;
+    }
   } else {
     mShSurfClient = gl->Screen()->Front();
     if (mShSurfClient && mShSurfClient->GetAllocator() &&