Don't fire mousemove when panning.

[mourner]

cc @jfirebaugh. Previously dragPan handler didn’t actually have an active property to track its state.

Also, dragRotate now becomes active when user actually starts dragging, not on mousedown.

[jfirebaugh]

@mourner Do you recall the rationale for not firing mousemove events during drag gestures?

[mourner]

@jfirebaugh as far as I recall, mousemove was supposed to reflect mouse position changing its location on the map (rather than screen position). When we drag-pan the map, the mouse stays in the same location (it's the location that moves).

[jfirebaugh]

...unless the map is up against the maximum Web Mercator latitude, or constrained by maxBounds, in which case the location does change.

I'm revisiting this due to #3290 and inclined to say that a map mousemove event should be fired for every DOM mousemove event, without exceptions.