Door markers aren't updated correctly

When teleporting from an interior to an exterior with no door markers in the map grid, the interior's door markers remain visible.

After visiting a sufficiently large interior, it should be possible for the exterior map to render door markers for unrelated exteriors.

Door markers outside the active grid but inside the rendered map area / view distance are not always drawn.

The nodes in LocalMapBase::mExteriorDoorsByCell aren't removed until a save is loaded, "leaking" a small amount of memory.

Morrowind.exe only redraws door markers when a cell enters the active grid / pannable map area. This means that doors enabled by scripts won't show on the map until the player leaves and reenters the area. OpenMW is currently somewhat more aggressive about redrawing these.