await cancel_flight in skip_flight

[jaredkhan]

Fixes a bug whereby skipping the animation would sometimes drop you in the wrong place.

e.g. on my machine, on the main tree...

onezoom.controller.leap_to(426632)
// then...
onezoom.controller.fly_on_tree_to(null, -237600)
// then in the first seconds of the flight...
onezoom.controller.leap_to(-237600)

lands me in a blank page.

And in fact it still does in this PR, but now the skip animation doesn't call those functions like that because it waits for the previous flight to be cancelled.

We can likely make those functions work directly, by having each one wait for the flight to be cancelled, hence the draft, this is just where I got to this weekend.