Implement EXT_discard_framebuffer in D3D11 renderer Change-Id: I0e39a196796813203c841c46aa2a6b89957f6051 Reviewed-on: https://chromium-review.googlesource.com/261355 Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>