Improve game_check_goal implementation slightly.

Improve game_check_goal implementation slightly.

It seems that, at least on x86, GCC can generate very slightly more
compact code by "expanding" the goal bitmap to match the game format,
rather what is currently done ("shrinking" the game bitmap to match
the goal format).

Add a sanity test case that checks the result of this function, which
makes such a change straightforward to do.