Implements a transition table from Pipeline Cache entry to
state change neighbouring Pipeline Cache entries. We use
a 64-bit mask to do a quick scan over the pipeline desc.
This ends up being a lot faster than doing a full hash
and memcmp over the pipeline description.

Note that there could be future optimizations to this design.
We might keep a hash map of the pipeline transitions instead
of a list. Or use a sorted list. This could speed up the search
when there are many transitions for cache entries. Also we could
skip the transition table and opt to do a full hash when there
are more than a configurable number of dirty states. This might
be a bit faster in some cases. Likely this will be something we
can add performance tests for in the future.

Documentation is also added in a README file for the Vulkan back
end. This will be extended over time.

Improves performance about 30-35% on the VBO state change test.

Bug: angleproject:3013
Change-Id: I793f9e3efd8887acf00ad60e4ac2502a54c95dee
Reviewed-on: https://chromium-review.googlesource.com/c/1369287
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>