We emulate RGB8 in D3D11 with RGBA8 textures. Make sure when we blit
that we don't copy the unused texture channel.

BUG=angleproject:1245
TEST=dEQP-GLES3.functional.fbo.blit.*

Change-Id: I805132fda984860a6d84ad4c1fc7169973938df9
Reviewed-on: https://chromium-review.googlesource.com/316010Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>