[ios] Add MGLMapView.showsScale to control scale bar visibility

[friedbunny]

• Adds MGLMapView.showsScale as the recommended way to show the scale bar.
• Adds IBInspectable for scale bar visibility.
• Fixes scaleBar remains hidden until the map's visible region changes #11302, where the scale bar was not visible until a camera change event. See note about scaleBar.hidden below.
• Includes two changelog entries, because I couldn’t see a concise way to combine the new property with the bug fix.

This isn’t technically breaking — setting MGLMapView.scaleBar.hidden = NO will still work, but is no longer the recommended approach.

Setting the scale bar view’s hidden property at initialization will now properly show that view when the map loads (because of the new updateScaleBar call in cameraDidChangeAnimated:), but setting this property later will still cause it not to be visible until a camera event occurs. The new showsScale property avoids this issue by calling updateScaleBar itself.

/cc @frederoni @1ec5 @fabian-guerra

[fabian-guerra]

Thank you for fixing this. It may be necessary that you retarget to release-agua for it being included in the next path release.

This is what I reviewed.

• Class/methods documentation according to MGLMapView.showsScale changes.
• Code readability.
• Code doesn't have side effects.

[friedbunny]

Thanks for the review, @fabian-guerra. These changes would be easy to retarget at release-agua, but I didn’t do that initially because a “soft deprecation” (i.e., changing the recommended way of showing the scale bar without breaking the old way) feels more appropriate for a semver minor (or major) release.

I’m not obstinately in that camp, though, so if folks feel like we should put this in the next v3.7.x patch release I can go ahead and retarget this PR.

[1ec5]

Per #11302 (comment), I’m wondering if we aren’t papering over a subtle bug with this change.

[friedbunny]

@1ec5 There may be an issue there, but I believe it’s incidental to the problems solved here:

• The initial visibility of the scale bar should not be tied to any camera change event. Making sure the map gets cameraIsChanging on its first load would not fix the case where a developer enables the scale bar after map initialization.
• We need to update the scale bar both on cameraIsChanging and cameraDidChange, as the final value may be different than the last reported intermediate value.

[friedbunny]

Added tests to verify that the scale bar is appearing when it should.