StateManagerGL: Use arrays to store texture and sampler bindings. The improved cache locality improves performance by 5-10% on command_buffer_perftests. BUG=angleproject:2188 Change-Id: I6cce95481e2b1ce2605bce19b0a29d56718613a1 Reviewed-on: https://chromium-review.googlesource.com/1156853 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>