Packing and referencing this structure was causing unnecessary draw
call overhead. This improves performance on all the back-ends. Impacts
the GL back-end the most.

In total this patch series reduces overhead by up to 5%.

Bug: angleproject:2933
Change-Id: Ief416ab874e481baf960d02965978a311214a146
Reviewed-on: https://chromium-review.googlesource.com/c/1299477
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>