Add specification for ANGLE_multiview 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/559089 Reviewed-by: Olli Etuaho <oetuaho@nvidia.com> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>