Patch for keeping the box in OverView map always in the center

[jachym]

Someone (me) could need, that the box, displaying current extend in the map, is always set to center. This little patch introduces new APIProperty to OpenLayers.Control.OverviewMap, which can turn this feature on.

[jachym]

New APIProperty, which can force always recenter the box in overview map

[tschaub]

(In [8805]) Add an autoPan property to the overview map. Set this to true if you want the extent marker to return to the center of the overview map after you drag it. Thanks for the patch jachym. r=me (closes #1770)

[crschmidt]

With this change (r8805), the OverviewMap fails tests in IE6, IE7, and Safari, due to the latitude not matching after a drag. I don't see why this is, but reopening as a reminder to trac this down.

[tschaub]

what should have been

[tschaub]

I tested with the original patch, then started messing with things to see the example difference. Default should be false (as the docs say). Apologies for the sloppy commit.

[tschaub]

(In [8841]) Correcting what was a mistake in r8805. Original patch was good. The autoPan property should default to false. Apologies for the failing tests. r=crschmidt (closes #1770)