Fix zero texture application. Patch https://chromium-review.googlesource.com/#/c/227711/ would allow NULL Textures sometimes from State::getSamplerTexture. Fix this by always setting the zero "Default" textures instead of NULL in the State. This was breaking the ES2-CTS test 'framebuffer_objects' on Windows. BUG=angle:826 Change-Id: Ie08a89cff0555f21c769759e0c0ed73456a2f91c Reviewed-on: https://chromium-review.googlesource.com/228275 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Shannon Woods <shannonwoods@chromium.org>