Only check for moving draggable in canvas renderer when map is draggable

[klaftertief]

This is a fix for #3941

[yohanboniface]

This becomes a bit hard to follow, I wonder if we should not have a method on the L.Draggable object to make this more readable.
Something like this._map.dragging.moving() that would do both checks.

[yohanboniface]

Also, this._map._dragging, inheriting from L.Handler has a enabled() method that should be used here instead of private property.

[mourner]

I agree. Should be a simple change, but let's look if we can update similar places to use the new method.

[klaftertief]

Yep. I'm happy to update the pull request later.

[yohanboniface]

Good catch! See code comments :)

[klaftertief]

Ok, I just added a moving() method to the MapDrag handler, just like the moved() method there.
I didn't find any other occurrences of the _moving property, except in the Draggable class itself.