Vulkan: Let RewriteInterpolateAtOffset use FlipRotateSpecConst Instead of using uniform, use flip/rotation specialization constant when available. This allows matrix multiplication calculated at compile time instead of shader execution time. Bug: angleproject:4901 Bug: angleproject:3589 Change-Id: I5a8a82b1cffbbbeba2617cb7e9a0ec38f9984d50 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2530457 Commit-Queue: Charlie Lao <cclao@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Ian Elliott <ianelliott@google.com>