Instead of using 8 bytes per vertex we can reduce the space used for
the divisor to 8 bytes. For larger values than 255 we can emulate the
divisor by unrolling the draw call. We will likely need to do this
in any case for instanced draws when the instancing extension isn't
available.

The tighter packing will allow for us to move the viewport and scissor
back into the pipeline description. It seems this is much faster than
using dynamic state. Every state change that would pull in a new
Pipeline would need the viewport and scissor re-applied. It seems
these driver calls are costly.

Does not improve perf significantly but enables future improvements.

Bug: angleproject:3013
Change-Id: I1a41c3acadc6fbd47c7a7b961c706e82f78de936
Reviewed-on: https://chromium-review.googlesource.com/c/1390358
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>