Merge zero textures into the state texture map.

We can treat these textures the same as the rest, saving a bit of
code and complexity cost.

BUG=angle:789

Change-Id: I14b426c817cbe3c5bb737410378b6a80e274689b
Reviewed-on: https://chromium-review.googlesource.com/227711
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>