These pack very well in some structs. Support is required so the
templates work as expected.

Also packs the Shader map into 8 bits. This includes a workaround for
an unusual warning generated by MSVC.

Bug: angleproject:2462
Change-Id: I804d1b9370b3951343718385210dec7d37700d73
Reviewed-on: https://chromium-review.googlesource.com/1091135
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>