The patch extends the behavior of
SH_SELECT_VIEW_IN_NV_GLSL_VERTEX_SHADER so that either the viewport
or layer is selected based on the value of the internal uniform variable
MultiviewRenderPath.

BUG=angleproject:2062
TEST=angle_end2end_tests
TEST=angle_unittests

Change-Id: Ia311b12b1fed642dac78eba8732e2535242f34fd
Reviewed-on: https://chromium-review.googlesource.com/615260
Commit-Queue: Martin Radev <mradev@nvidia.com>
Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>