gallium: implement full reference counting for vertex/fragment programs

Use _mesa_reference_vert/fragprog() wherever we assign program pointers.
Fixes a memory corruption bug found with glean/api2 test.
Another memory bug involving shaders yet to be fixed...

Picked from gallium-0.1

src/mesa/state_tracker/st_context.c

@@ -158,6 +158,9 @@ static void st_destroy_context_priv( struct st_context *st )
 {
    uint i;
 
+   st_reference_fragprog(st, &st->fp, NULL);
+   st_reference_vertprog(st, &st->vp, NULL);
+
    draw_destroy(st->draw);
    st_destroy_atoms( st );
    st_destroy_draw( st );