The patch adds the specification for the ANGLE_multiview extension
which enables efficient multiview rendering. The views can be laid
out side-by-side onto a 2D texture or rendered on separated layers of
a 2D texture array.

BUG=angleproject:2062

Change-Id: I4edd3ab0cfbb3906d90c2a5667d8d1f99f291f1c
Reviewed-on: https://chromium-review.googlesource.com/559089Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>