In some instances running a dispatch call could lead to accessing an
invalid VAO pointer. This seemed to be tied to switching Contexts.
Fix the bug by invalidating the cache pointers on Context switch. Note
that it seems this bug can only affect ES 3.1+ Contexts.

Bug: angleproject:3349
Change-Id: Ib712bcc29215b3fd1c8b7eb0cbd70dea649b3cfc
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1594289Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>