The OVR_multiview specification states that gl_ViewID_OVR is visible
at each pipeline stage. Previously to this patch the ViewID_OVR
impostor was declared only in the vertex shader and occurrences of
gl_ViewID_OVR in the fragment shader were not being handled. The
patch addresses the issue by declaring the ViewID_OVR variable as
a vertex output in the vertex shader and as a fragment input
in the fragment shader.

BUG=angleproject:2062
TEST=angle_unittests

Change-Id: I895953e81d3632d9bb873e8ac081fdf36f63f6b7
Reviewed-on: https://chromium-review.googlesource.com/559337
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>