glsl: be more careful about counting varying vars in the linker

glsl: be more careful about counting varying vars in the linker

Previously, we were counting gl_FrontFacing, gl_FragCoord and gl_PointCoord
against the limit of varying variables.  This prevented some valid shaders
from linking.

The other potential solution to this is to have the driver advertise
more varying vars or set the GLSLSkipStrictMaxVaryingLimitCheck flag.
But the above-mentioned variables aren't conventional varying attributes
so it doesn't seem right to count them.

Reviewed-by: Eric Anholt <eric@anholt.net>