Mitigate overhead of walking all loaded submaps for vehicle processing.

[kevingranade]

The code is inappropriately walking all loaded submaps in order to process vehicle power production and consumption.
This mitigates the overhead by skipping most of the processing in the common case when a submap has no vehicle.

Summary

SUMMARY: Bugfixes "Mitigate out of control vehicle processing overhead."

Purpose of change

As reported in #36453 the game is suffering from increasing per-turn overhead the more map area is explored between saves.
The cause of this is this chunk of code that updates idle state.

Cataclysm-DDA/src/game.cpp

Lines 1520 to 1531 in 64b5e04

	for( auto &elem : MAPBUFFER ) {
	tripoint sm_loc = elem.first;
	point sm_topleft = sm_to_ms_copy( sm_loc.xy() );
	point in_reality = m.getlocal( sm_topleft );
	submap *sm = elem.second;
	const bool in_bubble_z = m.has_zlevels() || sm_loc.z == get_levz();
	for( auto &veh : sm->vehicles ) {
	veh->idle( in_bubble_z && m.inbounds( in_reality ) );
	}
	}

Describe the solution

This partially mitigates the problem by doing a very cheap check to skip most (I think about 80%) of the overhead caused by this issue, the remaining work is simply iterating over the tree.

Describe alternatives you've considered

I need to completely replace this mechanism with a list of vehicles on the map plus vehicles connected to them, which will not scale in the same way.

Testing

Insure vehicles are still processed, i.e. fuel and battery consumption.

[ZhilkinSerg]

sm is only initialized several lines below.

[kevingranade]

Just as well I have a better fix ready.