Fix rendering to, initializing and clearing textures that are backed by more channels in D3D11 than the GL format has.

Change-Id: I250c349254acf4e6f85d4eff58e2190edb1782ef
Reviewed-on: https://chromium-review.googlesource.com/183994Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Nicolas Capens <nicolascapens@chromium.org>
Commit-Queue: Nicolas Capens <nicolascapens@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>