glsl: Pass stage to add_interface_variables().

glsl: Pass stage to add_interface_variables().

add_interface_variables is supposed to add variables from either the
first or last stage of a linked shader.  But it has no way of knowing
the stage it's being asked to process, which makes it impossible to
produce correct stagerefs.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>