softpipe: implement flatshade_first for line setup

src/gallium/drivers/softpipe/sp_setup.c

@@ -1051,7 +1051,10 @@ setup_line_coefficients(struct setup_context *setup,
    float area;
 
    /* use setup->vmin, vmax to point to vertices */
-   setup->vprovoke = v1;
+   if (softpipe->rasterizer->flatshade_first)
+      setup->vprovoke = v0;
+   else
+      setup->vprovoke = v1;
    setup->vmin = v0;
    setup->vmax = v1;