glsl: Add a lowering pass to remove reads of shader output variables.

glsl: Add a lowering pass to remove reads of shader output variables.

This is similar to Gallium's existing glsl_to_tgsi::remove_output_read
lowering pass, but done entirely inside the GLSL compiler.

Signed-off-by: Vincent Lejeune <vljn@ovi.com>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>