Use - 0.5 instead of + -0.5 in FlipGLPointCoord.

Bug: angleproject:2673
Change-Id: I819dcc45f9098ea64085eb71af8920fe99b67c62
Reviewed-on: https://chromium-review.googlesource.com/1194454Reviewed-by: Tobin Ehlis <tobine@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>