Support disable() mid drag or rotate gesture

[jfirebaugh]

Moves DragPanHandler and DragRotateHandler closer to a state machine based implementation, so that, when disabled, they can perform the appropriate actions in any state.

These two handlers have four states:

• disabled: The handler is disabled, and should not respond to any input.
• enabled: The handler is enabled but has not started receiving any input.
• pending: The handler has received a mousedown or touchstart event, but not a subsequent move event.
• active: The handler has received a mousedown or touchstart event and at least one subsequent move event.

Disabling a handler in the enabled state doesn't generally require anything special. Disabling a handler in the pending state requires unbinding some event listeners. Disabling a handler in the active state requires unbinding some event listeners, sending appropriate *end events, and not triggering an inertial camera animation.

Fixes #2419.

[anandthakker]

Just noting: because there's a state called 'active', I (briefly) found it confusing to have a method called _deactivate that doesn't affect this.state... but I can't think of any better names.