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>