glsl/nir: Respect GLSLFrontFacingIsSysVal
If GLSLFrontFacingIsSysVal is not set, drivers should not front-facing sysvals in, full stop. Only lower here conditionally.
Signed-off-by: Alyssa Rosenzweig alyssa.rosenzweig@collabora.com
As discussed on IRC. Cc @jekstrand @airlied