glsl: rename lower_clip_distance to lower_distance.

This just renames the file in anticipation of adding cull lowering,
and renames the internals.

Signed-off-by: Tobias Klausmann <tobias.johannes.klausmann@mni.thm.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>
2 files changed