draw/so: respect leading/provoking vertex info

we were ignoring leading/provoking vertex settings which was
breaking decomposition of some strips.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>

src/gallium/auxiliary/draw/draw_so_emit_tmp.h

@@ -10,7 +10,7 @@
    const unsigned prim = input_prims->prim;                       \
    const unsigned prim_flags = input_prims->flags;                \
    const boolean quads_flatshade_last = FALSE;                    \
-   const boolean last_vertex_last = TRUE;                         \
+   const boolean last_vertex_last = !so->draw->rasterizer->flatshade_first;  \
    do {                                                           \
       switch (prim) {                                             \
       case PIPE_PRIM_LINES_ADJACENCY:                             \