This will dramatically speed up performance on 64-bit systems due to
the efficiency of angle::BitSet64. Improves performance of the GL
back-end on benchmarks quite a bit.

Squashes the Pack and Unpack states together. Also moves the current
value dirty bits to a single dirty bit, with a separate set for each
attribute. Also squashes a multisample dirty bit.

Also fixes an incorrect dirty bit in StateManagerGL.

We may want to implement a semi-fast version of BitSet64 for 32-bit
systems if we find performance is not satisfactory.

BUG=angleproject:2188

Change-Id: I4616782bf75413252ede3e3ac752b9ccdb9aab49
Reviewed-on: https://chromium-review.googlesource.com/722423Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>