This extends the current y-flip that's done for gl_FragCoord.
Now, gl_FragCoord is rotated and then flips both the x and y-axis.

Bug: angleproject:4643
Bug: b/156395519
Change-Id: Iada8680eda8322f7382ff242e4a9422a66114f05
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2210936Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Ian Elliott <ianelliott@google.com>