Move theme internals into anonymous namespace, misc cleanups

[ephphatha]

Was looking at adding a helper for checking if a point matches a given regionId however I don't really want to introduce an unavoidable function call by using the same pattern as IsTileSolid and defining it in path.cpp. Probably move all that into a new header for dungeon tile checks?

This was the intent for CheckThemeObj5 but omitted for now:

bool IsTileInRegion(Point position, int8_t regionId)
{
	return InDungeonBounds(position) && dTransVal[position.x][position.y] == regionId;
}