Animate the user dot

[1ec5]

Previously, the user dot would blit across the screen, and MGLMapView would try to keep up but would tragically remain one step behind. With this PR, the user and MGLMapView waltz happily together.

• Animate the user dot using UIView block-based animation
• Make the user annotation callout view track the user dot
• Make the viewport track the user dot more smoothly (split out into Affix user dot on screen in user tracking mode #3589)
• Keep the user dot from drifting when panning the map view with gestures (more or less addressed by Disable implicit animation of user dot bounds #3683)

Depends on #3589. Fixes #1041. Ref #1125.

[1ec5]

I’d like to make user tracking smoother by sticking the user dot in the center of the screen and moving the viewport instead of the user dot. Unfortunately, -notifyMapChange: is called way too often, even from within mbgl::Map, preempting the animation by atomically moving the user dot to the final position.

[1ec5]

Another problem is that Transform::setLatLngZoom() hard-codes an ease-in/ease-out animation, whereas a linear animation is called for when animating user tracking.

[incanus]

Unfortunately, -notifyMapChange: is called way too often

#1431

Calling these sorts of methods per-frame is not useful to users

/cc @kkaefer

[1ec5]

#2204 complicates things here too.

[1ec5]

I split the user tracking case out into #3589.

[1ec5]

I rebased this branch atop #3589. It looks pretty good, other than a brief misplacement of the callout after selecting a user dot that’s in motion.

/cc @friedbunny @zugaldia