radv: Handle clip+cull distances more generally as compact arrays.

Needed for https://gitlab.freedesktop.org/mesa/mesa/merge_requests/248

 .

That MR keeps the clip and cull arrays split.

So we have to handle
 - compact arrays with location_frac != 0
 - VARYING_SLOT_CLIP_DIST1

Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
(cherry picked from commit 1ef28556)