How to set the rotation center of an MKAnnotationView
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I've got a problem with my `MKAnnotationView`s when `MKUserTrackingModeFollowWithHeading` is enabled on the MKMapView.

I positioned my images using the centerOffset property of the MKAnnotationView. Specifying the coordinates of the pin's tip relative to the coordinate system at the center of the image is somewhat counter-intutive, but I came up with the following formula:

    annotationView.centerOffset = CGPointMake(imageWidth/2.0 - tipXCoordinate, imageHeight/2.0 - tipYCordinate);

This works fine for zooming the map in and out. The tips of the pins keep their relative position on the map.

However, when I enable `MKUserTrackingModeFollowWithHeading`, it won't work anymore. The Pins rotate around the center of the image, instead of the tip. So when the map rotates, the tips do no point to the locations they are supposed to annotate.

I've played around a bit with the `frame`and `center`properties of the `MKAnnotationView`, but I feel, they are having no effect on the alignement of the pins whatsoever.

Interestingly, the `MKPinAnnotationView` does not seem to use `centerOffset` at all, but a shifted `frame` instead. However, I was unable to reproduce this. Changing the `frame` of my custom view did not move it at all.

Thanks for any insights you can provide :-)