Some improvements for BlitFramebuffer. This change can 1. Skip color blitting when draw buffers are disabled. 2. Skip depth/stencil blitting when depth/stencil buffer has no image. 3. Return early if possible. 4. Move the logic above from D3D backend to common part. BUG=angleproject:1677 Change-Id: I1662d214b72fb8caa4a95c86d9b8a67984b6071a Reviewed-on: https://chromium-review.googlesource.com/423135 Commit-Queue: Yunchao He <yunchao.he@intel.com> Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>