meta/blit: Save and restore the sampler using gl_sampler_object instead of GL API...

meta/blit: Save and restore the sampler using gl_sampler_object instead of GL API object handle

Some meta operations can be called recursively.  Future changes (the
"Don't pollute the ... namespace" changes) will cause objects with
invalid names to be used.  If a nested meta operation tries to restore
an object named 0xDEADBEEF, it will fail.

v2: Add a comment explaining why samp_obj_save is set to NULL in
_mesa_meta_fb_tex_blit_begin.  This came out of review feedback from
Jason.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>