Don't call onDrag when multiple fingers are touching the map

[trygveaa]

Calling onDrag when multiple fingers are touching the map causes a delay when you start pinch zooming. This is because the pinch is interpreted as a tap in the beginning, which causes stopPropagation to be called.

I think dragging is an action you only do with one finger, so therefore I think not calling onDrag is the correct solution to this.

I'm not sure how to test this, as I haven't figured out how to check if events are called. Are there any existing tests doing such checks?

Fixes #962

[underbluewaters]

I've used this approach in my own fork to avoid wrecking a shape that's in direct_select mode while pinch-zooming on a mobile device. I'm finding editing on mobile to be pretty impossible without that change.