Tap gesture recognizer should fail unless annotation is nearby

[1ec5]

MGLMapView’s built-in UITapGestureRecognizer should set its state to UIGestureRecognizerStateFailed as soon as it discovers that no annotations are nearby. Combined with #2278, the developer would be able to install their own “fallback” tap gesture recognizer that handles taps when the built-in gesture recognizer, meant for annotation selection, has nothing to select.

/cc @danpat @incanus

[danpat]

I hit this. I have annotations on the map, but I would also like to allow the user to tap elsewhere on the map and perform a feature lookup using visibleFeaturesAtPoint:.

[1ec5]

Note that the recognizer should not fail if an annotation is already selected and tapping closes the annotation’s callout view.

[1ec5]

Fixed in #7246.