WIP: move c to vs_vec4_visitor.  TODO: split out "c->prog_data." -> "prog_data->" changes.
i965: Remove unused userclip flags.

brw_vs_prog_data::userclip hasn't been used since commit f0cecd4
(i965: Move VUE map computation to once at VS compile time).

brw_gs_prog_key::userclip_active hasn't been used since commit 9f3d321
(i965: Make the userclip flag for the VUE map come from VS prog data).