The gl_PointCoord.y coordinate needs to be flipped
around the X axis in Vulkan to have correct results.

Bug: angleproject:2457

Change-Id: I0d87ad28366623c2be0867c610cc35678a4af43f
Reviewed-on: https://chromium-review.googlesource.com/1090824Reviewed-by: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Commit-Queue: Luc Ferron <lucferron@chromium.org>