draw: flush when setting stream-out targets

We need to re-prepare the middle-end state to pick up changes to this
state to react correctly to pausing/resuming stream-out. So let's add a
flush here.

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Fixes: ec8cbd79ac "draw/softpipe: EXT_transform_feedback support (v2)"
Reviewed-by: Roland Scheidegger <sroland@vmware.com>

src/gallium/auxiliary/draw/draw_context.c

@@ -950,6 +950,8 @@ draw_set_mapped_so_targets(struct draw_context *draw,
 {
    int i;
 
+   draw_do_flush( draw, DRAW_FLUSH_STATE_CHANGE );
+
    for (i = 0; i < num_targets; i++)
       draw->so.targets[i] = targets[i];
    for (i = num_targets; i < PIPE_MAX_SO_BUFFERS; i++)