Add compass image as a sublayer

[julianrex]

Fixes #500

Changing the transform on the compass button triggers -[UIView layoutSubviews] on iOS 14. This PR add a sublayer for the compass image, keeping the original UIView's transform intact.

The original issue in #500 is not fully resolved however, as the scale bar causes a similar issue (to a lesser extent).

[julianrex]

cc @MaximAlien

[lloydsheng]

Since we have added an imageLayer, does MGLCompassButton still need to inherit UIImageView ?
I think there may be a better solution to this issue. In order to avoid -[UIView layoutSubviews] being called, Do we have to add an extra layer to the subview?

[julianrex]

Since we have added an imageLayer, does MGLCompassButton still need to inherit UIImageView ?

Good catch; because it's called "Button" I had assumed it was a UIButton. I'll change to a plain UIView. Let's keep it as the UIImageView for the time-being, as it could be argued changing it would be a SEMVER major change.

I think there may be a better solution to this issue. In order to avoid -[UIView layoutSubviews] being called, Do we have to add an extra layer to the subview?

Something other than the compass view needs to have its transform changed. This works for both a subview and a sublayer. I chose a sublayer since it won't play a role in auto-layout.

[MaximAlien]

Would it be possible to back-port current fix to version 6.2 as well?

[captainbarbosa]

Look good, just one question - does this change any accessibility-related functionality?

[captainbarbosa]

For next time, I'd maybe consider splitting this into a separate PR.

[julianrex]

does this change any accessibility-related functionality?

No, this should be a visual change only.