glsl: clean up and fix bug in varying linking rules

glsl: clean up and fix bug in varying linking rules

The existing code was very hard to follow and has been the source
of at least 3 bugs in the past year.

The existing code also has a bug for SSO where if we have a
multi-stage SSO for example a tes -> gs program, if we try to use
transform feedback with gs the existing code would look for the
transform feedback varyings in the tes stage and fail as it can't
find them.

V2: Add more code comments, always try to remove unused inputs
to the first stage.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>