Show how to download a non-rectangular offline pack

[1ec5]

Now that mapbox/mapbox-gl-native#11447 has landed, the “Download an offline map” example should show how to download a non-rectangular region or at least hint at the possibility of doing so by replacing MGLTilePyramidOfflineRegion with MGLShapeOfflineRegion. It would be particularly helpful to show how to add an offline pack that follows a route or the path of a camera animation.

/cc @captainbarbosa @riastrad

[captainbarbosa]

We should still do this.