Made locking a little tighter and event processing a little lazier.
Fixed a bug where null would be returned instead of an empty list if a route had no vehicles in service.
Added markers for vehicles on visible routes.
Refactored to handle potentially-null vehicle IDs a little more gracefully.
Added preliminary support for inferring vehicle bearings.
Converted "math degrees" to "compass degrees" for inferred vehicle bearings.
Rotate vehicle markers to reflect bearing.