draw: more flatshade_first changes

- Reduce the number of changes to the normal vertex ordering
- Assume that the hardware knows how to do this in the standard case.
- Add support to the passthrough vcache path.

src/gallium/auxiliary/draw/draw_pt.c

@@ -191,7 +191,7 @@ boolean draw_pt_init( struct draw_context *draw )
    if (!draw->pt.middle.fetch_emit)
       return FALSE;
 
-   draw->pt.front.vcache = draw_pt_vcache();
+   draw->pt.front.vcache = draw_pt_vcache( draw );
    if (!draw->pt.front.vcache)
       return FALSE;