transit-vehicles-overlay: no popup / no highlight

[fpurcell]

don't allow the map hover to highlight if there isn't a popup (and/or no forced highlight prop?).
don't want icons to look like there's something under a marker

[fpurcell]

well, that said, if click == select, then we'd want that highlight to be there, even tho no popup or tooltip, etc... Hmmm