Prefetch destination tiles when animating a viewport change

[1ec5]

When you animate a significant change to the map view's visible region, you always arrive at the destination before the destination tiles have loaded. mbgl should determine which tiles will be needed at the destination and prefetch them as soon as the animation begins.

/cc @kkaefer @incanus

[1ec5]

With the default animation duration of 300 ms, prefetching might not make a huge difference. But now you can specify a custom duration for -setCamera:withDuration:animationTimingFunction:, and doing so would make a lot of sense for a flyTo()-style animation – in that case, you’d definitely want to calculate which tiles matter and start loading them as soon as the animation begins, rather than on demand.

[stale]

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