The patch modifies the vertex shader's AST to declare and initialize
the global variables ViewID_OVR and InstanceID. Every occurrence of
gl_ViewID_OVR gets replaced by ViewID_OVR and initialized in main
with a value dependent on gl_InstanceID and the number of views.
To guarantee correct results for instanced rendering, each occurrence
of gl_InstanceID is replaced with InstanceID and initialized similarly.

BUG=angleproject:2062
TEST=angle_unittests

Change-Id: I48be688605b5af869bc370758e70ccc209ea4419
Reviewed-on: https://chromium-review.googlesource.com/548596
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>