Opening control center during pan animation causes map to disappear

[bmeine]

I've only tested this on a physical device with iOS 7.1 using the sample app.

To reproduce:
-Zoom in far enough that you have plenty of panning room (max or close to it)
-Start panning very fast from bottom to top
-During one of the rapid upward swipes, start your finger below the screen so it brings up the control center
-The control center opens and the map is gone...the Mapbox logo and attribution are still there

It seems to only happen if the control center is opened while the panning animation is still running.

[1ec5]

MGLMapView throws up a snapshot of the map upon receiving a UIApplicationWillResignActiveNotification or UIApplicationDidEnterBackgroundNotification, to avoid making OpenGL calls in the background (which causes crashes). The snapshot taken during animation can sometimes be behind or omit some tiles.

We added the UIApplicationWillResignActiveNotification because we were still seeing background-drawing crashes. However, that was probably #1460, fixed in #1869, so we should double-check whether snapshotting on UIApplicationWillResignActiveNotification is still required.

[1ec5]

(The same behavior occurs with Notification Center or any modal alert.)

[incanus]

Another option here would be to cancelAnimations() upon receiving the signal?

[1ec5]

#2309 is an attempt at fixing this issue.