This new driver uniform allows us to dynamically decide if we
flip the y coordinate of the gl_PointCoord variable.

We only flip the coordinate if we render downside-up, and otherwise
we cancel the flipping since the y coordinate doesn't need
modification anymore.

This completes all end2end and deqp gles2 tests success when viewport
flipping is enabled on Windows Nvidia.

Bug: angleproject: 2673
Change-Id: I570e01ef4965933428375f6ebd8eac83ef16fbff
Reviewed-on: https://chromium-review.googlesource.com/1133584
Commit-Queue: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>