Disable EXT_depth_textures on AMD D3D9 devices.

BUG=angle:839

Change-Id: Ia8c24db98a31141994e22b25efbdfddaa4eced49
Reviewed-on: https://chromium-review.googlesource.com/231996
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>
Optimize LoadL8ToRGBA8.

We can save some work by re-using the color value in a temporary.

BUG=angle:842

Change-Id: Ib64b34ca7bc77c73368e159527ad84419079f958
Reviewed-on: https://chromium-review.googlesource.com/231997
Reviewed-by: Bruce Dawson <brucedawson@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>