[core] calling flyTo with 0 duration does not call transitionFinishFn

[julianrex]

If you call mbgl::Map::flyTo with a zero duration, then it shortcuts to jumpTo and then easeTo with default (empty) AnimationOptions, meaning that the AnimationOptions.transitionFinishFn will not get called.

The transitionFinishFn should be passed through to easeTo.

(On iOS this results in any passed completion handler not being called).

Tested in ios-v5.3.0-beta.1 (detected during testing of #15122)

[julianrex]

transitionFinishFn should also be called in this return:

if (std::isnan(zoom) || std::isnan(bearing) || 
     std::isnan(pitch) || state.size.isEmpty()) {
    return;
}

for both flyTo and easeTo.

For that matter, transitionFinishFn should probably take a parameter indicating whether the transition completed successfully.