Refs #1256 - Fix some erronenous tracking mode resets

[friedbunny]

• Adds private preservingTracking: version of setCenterCoordinate:zoomLevel:animated:
• Changes zoomToSouthWestCoordinate:northEastCoordinate:animated: to above preservingTracking:, fixes an untended tracking mode reset
• Removes tracking mode reset from resetNorthAnimated:
  • handleCompassTapGesture: was intending to simply bump down to MGLUserTrackingModeFollow if it was following with heading, but it was getting clobbered by resetNorthAnimated:.

Only fixes half of #1256.

[friedbunny]

May be better to move the tracking mode check from handleCompassTapGesture to resetNorthAnimated, though there are already plenty of ways to reset the tracking mode if a developer wanted to.

[jfirebaugh]

@1ec5 / @friedbunny Can you coordinate what remains to be done here and get this merged?

[1ec5]

@friedbunny, it looks like #1783 took care of the -zoomToSouthWestCoordinate:northEastCoordinate:animated: case. Do you think we still need a way to prevent resetting the user tracking mode generally? If so, that version of the method should be exposed publicly. Otherwise, this PR can be closed.

[friedbunny]

The resetNorthAnimated: tracking mode reset still needs to be removed, but I've broken that out into a new pull request (#1809). Closing here.