Remove 'pv' parameter from Line/Tri/Point funcs. The provoking vertex

Remove 'pv' parameter from Line/Tri/Point funcs.  The provoking vertex
is always the last vertex parameter.
Modify clipping to preserve pv colors.
Modify swrast and X11 driver to expect the pv in the last vertex
(was looking in the first vertex previously).
Remove all handling of flatshading from swrast_setup.

Allow drivers to override the unclipped render tabs in tnl_render_stage
directly.  (Like in 3.4).  Removed fxsimplerender stage.

Modified t_vb_rendertmp.h to remove the need for 'parity' arguments
in RENDER_TRI macros.