Automated OAMs management

Over the summer, I crafted plans for managing larger levels, that would use more than 128 hardware sprites (the OAMs in DS parlance) for all the monsters. Some of those plans use the notion of a "Gob Group", that could be used to spawn several objects at once, at level-specific locations by having them statically placed (unlike shoot-able "dyngobs") but wouldn't be active unless their group is enabled.

Browsing through the game engine code for another purpose on my cybook, it struck me that it would be so easy to have lazy allocation of the hardware sprites as we approach on-screen area and early recycling of those OAMs when we're going far enough from that area. It's now coded. Funny enough, it doesn't affect activation and movement of GOBs in any way: only whether they have hardware sprites assigned to them or not.

done: find a way for Bilou to keep its priority (displays over monsters)