D3D11: Add RGB10_A2 as a supported EGL pbuffer config format. Update EGLWindow to select configs that exactly match the requested config. Too many tests make assumptions about the default framebuffer's format. BUG=angleproject:1662 Change-Id: I4cf8aedf51013ca0f3f40f6bbd40f24a4a90561f Reviewed-on: https://chromium-review.googlesource.com/419681 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>