Vulkan: Fix flipXY for 180 degree prerotation and spec consts The multiplier used for flipXY in this configuration was incorrect. Bug: angleproject:5348 Bug: angleproject:5349 Bug: angleproject:5350 Change-Id: I0b3771f6a9da6ab2af02ed5ee5fe7b49d0415410 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3016768 Reviewed-by: Ian Elliott <ianelliott@google.com> Reviewed-by: Charlie Lao <cclao@google.com> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>