Vulkan: Clean up point coord translation. Use - 0.5 instead of + -0.5 in FlipGLPointCoord. Bug: angleproject:2673 Change-Id: I819dcc45f9098ea64085eb71af8920fe99b67c62 Reviewed-on: https://chromium-review.googlesource.com/1194454 Reviewed-by: Tobin Ehlis <tobine@google.com> Commit-Queue: Jamie Madill <jmadill@chromium.org>