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>