Only store zoomPan event in mousemove if it has a source event (closes #4706)
Only store zoomPan event in mousemove if it has a source event
(closes #4706)

d3 transitioned events don't have a source event,
causing mousemove to go null

openstreetmap/iD,morray/iD,digidem/iD,mapmeld/iD,AndreasHae/iD,kartta-labs/iD,kartta-labs/iD,mapmeld/iD,bagage/iD,digidem/iD,openstreetmap/iD,AndreasHae/iD,mapmeld/iD,ngageoint/hootenanny-ui,morray/iD,kartta-labs/iD,AndreasHae/iD,bagage/iD,ngageoint/hootenanny-ui,ngageoint/hootenanny-ui,openstreetmap/iD,bagage/iD