[ios] fixes #5036 draggable annotation views

[frederoni]

Fixes #5036

Setting MGLAnnotationView's draggable property to YES will make it draggable.
In order to make it stick to the location where it was dropped, you have to implement;
-mapView:didDragAnnotationView:toLocation:
and update the coordinate property of the associated annotation.

Made a few changes compared to previous PR which didn't take center offset into account.

• Circle offset is 0,0
• Rectangle offset is "bottom-left"

also updated doc in MGLMapViewDelegate, I hope it's more clear now.

/cc @1ec5 @boundsj @friedbunny

[1ec5]

The macOS build failed because this branch is based on release-ios-3.3.0, which doesn't have the OS X-to-macOS renaming from #5343. I've added a special Bitrise workflow for that release branch, but macOS builds will continue to fail for new PRs made against that branch.

@boundsj @friedbunny @kkaefer, maybe we should cherry-pick #5343 into the release branch to keep things sane. The macOS builds are important to iOS SDK development because the two SDKs share so much code.

[friedbunny]

I'm 👍 on cherry-picking macOS into the release branch.

[1ec5]

I’ve cherry-picked #5343 into the release branch; the build failure should go away once you rebase.