transit-vehicles-overlay: marker icon / zoom controls

[fpurcell]

currently, both the marker icons (simple rectangle, detailed vehicle) and the zoom levels are hard coded in the slot component.

3 options:

1. remove the simple rectangles and just go with the detailed images (best bet? far out zoom?)
2. adjust slot for more vehicle marker images and less simple rectangle
3. add more knobs to control the zoom / icon variables