Vulkan: Add entry points for GL_EXT_fragment_shading_rate Bug: angleproject:420310117 Change-Id: I8a8efad6cad810b2cb0b600106f6496070495605 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6620340 Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Alexey Knyazev <lexa.knyazev@gmail.com>