Add shader translator support for OVR_multiview The OVR_multiview and OVR_multiview2 extensions add gl_ViewID_OVR to shaders. gl_ViewID_OVR can be translated either as is in GLSL output or as a uniform by setting the SH_TRANSLATE_VIEWID_OVR_AS_UNIFORM compiler flag. If WebGL output is selected, the shaders will be validated according to proposed rules in the WEBGL_multiview spec. BUG=angleproject:1669 TEST=angle_unittests Change-Id: I19ea3a6c8b4edb78be03f1a50a96bfef018870d0 Reviewed-on: https://chromium-review.googlesource.com/422848 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>