This change is two-fold: it splits the compatible parts from the
RenderPass description, which allows for correct caching. It also
will allow the pipeline state cache to cache the compatible RenderPass
bits.

In order to do this we introduce a packed version of the RenderPass
description, which uses carefully aligned bit representations so we
can hash the descriptions easily using MurmurHash, and compare them
with memcmp.

Bug: angleproject:2163
Change-Id: I4179c0ff8762df81f29082168bd6e3056ceb1318
Reviewed-on: https://chromium-review.googlesource.com/815816
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>