meta: Unify the GLSL and fixed-function clear paths.

meta: Unify the GLSL and fixed-function clear paths.

The majority of _mesa_meta_Clear and _mesa_meta_glsl_Clear was the same;
adding a boolean for whether to use GLSL allows us to share most of it
without polluting either path too much.

Tested for regressions by hacking i965 to always use the non-GLSL path.

Cc: "10.2" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>