glsl: Set .flat for gl_FrontFacing

It is a boolean.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3237>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3237>
1 file changed