st/mesa: flip y coordinate of interpolateAtOffset for winsys

This fixes a few dEQP tests like

dEQP-GLES31.functional.shaders.multisample_interpolation.interpolate_at_offset.no_qualifiers.default_framebuffer

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Dave Airlie <airlied@redhat.com>
1 file changed