This includes a partial implementation of multisample texture
operations on the HLSL backend. It can't be fully tested yet, since
the API side isn't implemented.

BUG=angleproject:1442
TEST=dEQP-GLES31.functional.shaders.builtin_functions.texture_size.*
     (successfully compiles instead of crashing)

Change-Id: Ief782db28388a3f8fd8113cc86ce3c4f500f322a
Reviewed-on: https://chromium-review.googlesource.com/443264Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>