This will enable future optimizations that only update either the
applied vertex buffers or the input layout independenty. It also
makes debugging a bit easier since we can force an update of the
vertex buffer or input layout to diagnose potential state sync
bugs.

BUG=angleproject:1156

Change-Id: Ib375561528da539e136ff4fd02398b03c9caabb7
Reviewed-on: https://chromium-review.googlesource.com/627077Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>