This extends the current y-flip that's done for gl_PointCoord.
Now, gl_PointCoord is rotated and then flips both the x and y-axis
(similar to  gl_FragCoord).  The tests used to reproduce the
problem and the fix are:

angle_deqp_gles2_tests --local-output --gtest_filter=dEQP.GLES2/functional_shaders_builtin_variable_pointcoord* --use-angle=vulkan

angle_end2end_tests --local-output --gtest_filter=*GLSLTest.PointCoordConsistency*Vulkan*

Bug: b/157476696
Change-Id: Iada8680eda8322f7382ff242e4a9422a66114f07
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2216700
Commit-Queue: Ian Elliott <ianelliott@google.com>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>