* Remove indirection when getting CotextState from Context
* Reduce number of branches from 2 to 1 when checking for an InvalidEnum in FromGLEnum<DrawElementsType>
* Provide IndexRange constructor which doesn't initialize the member variables
* Reduce number of branches to 1 when checking for BasicDrawStateErrors for the fast path
* Remove a few branches and reduce math cost during the buffer range check in ValidateDrawElementsCommon

Bug: angleproject:2966

Change-Id: Ibdbed8a59ad9d7abce76622e9f507498e67ab997
Reviewed-on: https://chromium-review.googlesource.com/c/1392805
Commit-Queue: Markus Tavenrath <matavenrath@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>