Normalize angle of 360 degrees to 0 degrees

[friedbunny]

This fixes the compass not disappearing after it is reset, especially when returning from the background.

When interpolating across 180º, _normalizeAngle will return >180º. Then, trying to reset the bearing to 0º would result in interpolation to 360º. Because [MGLMapView updateCompass] expects bearing to equal 0º when the compass is done resetting, the compass was never disappearing.

Steps to reproduce

1. Turn on heading-follow location tracking in iOS demo app
2. Rotate device to a heading less than 180º (due south)
3. Lock device
4. While locked, rotate beyond 180º
5. Unlock device, tap compass to reset heading
6. Compass will reset, but not disappear because bearing is 360º, not 0º

You can also test this by setting a breakpoint in /src/mbgl/map/transform.cpp _normalizeAngle and doing this:

Start by moving to 171º from 196º:

(lldb) po _normalizeAngle(-2.999457083051174, 3.4266856461794291)
3.2837282241284118 = 189º

When it comes time to reset to 0º from 189º:

(lldb) po _normalizeAngle(0, 3.2837282241284118)
6.2831853071795862 = 360º

Which is the same as 0º, but not what we're looking for.

/cc @1ec5 @incanus

[1ec5]

In the course of refactoring the compass for #1496, I discovered another off-by-ε error that may be related.

[jfirebaugh]

Does this preserve the behavior that animated rotations from ø > 180º to 0º take the shortest path around the circle, i.e. to 360º?

[friedbunny]

@jfirebaugh Yes, at least in the case of the iOS compass.

[friedbunny]

Found an easier way to trigger the original bug: do the rotation gesture very hard.

[kkaefer]

Does this only happen for 180 degrees? If so, it looks like the culprit may be in util::wrap, because, the range should be exclusive in one end (preferably the high one), so it should normalize -180 <= x < 180, rather than -180 <= x <= 180.

[friedbunny]

Perhaps @mourner could weigh in on our use of util::wrap?

template <typename T>
T wrap(T value, T min, T max) {
    T d = max - min;
    return value == max ? value : std::fmod((std::fmod((value - min), d) + d), d) + min;
}

JS implementation is here.

[friedbunny]

This was fixed in #1829 by @1ec5.