Prevent overlapping mouse down events

[eddydg]

When handler drag event overlap, only trigger the drag mouse down event corresponding to the cursor style:

• move -> center handler mouse down event
• ns-resize , ew-resize -> radius handler mouse down event

Issue: https://github.com/smithmicro/mapbox-gl-circle/issues/80

[rajkdale]

I am facing same issue with overlapping clicks. When will this merge ?