This moves much of the math into cached values in the VertexAttribute
and VertexBinding.

Bug: angleproject:1391
Change-Id: I1b6c0553bf57fef864c27c5193c7dd7ca9b56f53
Reviewed-on: https://chromium-review.googlesource.com/1008274
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>