Emergency vehicles

* Rename the "LIGHT" flag to "CONE_LIGHT"
* Circular vehicle lights! They expose a weird lighting bug, but I'll cover that later. Just add CIRCLE_LIGHT to the part flag list.
* Flasher lights! Circular lights with EVENTURN or ODDTURN flags will only flash on even/odd turns respectively. (If both are omitted then the light doesn't flash - good for omnidirectional lights)
* Ambulances and police cars! They can spawn in cities. Sometimes with their flasher lights still on. Ambulances have a bed in back and a side opaque door.

Pretty sure I made it so I wouldn't break save compatibility.

Now, the bug: It seems that add_light_source doesn't handle it very well when two light sources overlap (thereby intensifying the light) and then reach the border of a submap. It's outside the scope of what I was doing, and I definitely didn't cause it (I think, anyway), so any investigation can go into another PR.