Initial centerpoint not preserved during subsequent map edits

Motivation

If you set an initial center point for a map, it is only saved correctly at the first save. Every later save of the map will change the position slightly, if you do not reset the center point.

How to reproduce:

• To easier see the error, go to Structure > OpenLayers and make sure, that "Preview map" is checked.
• Add a new map at Structure > OpenLayers > Maps > Add.
• Enter map title "Test".
• Switch to "Layers & styles" tab. Make sure that MapQuest OSM is active and set as default map (it actually doesn't matter, which map you choose, as long as some map is active).
• Switch to "Center & bounds" tab.
• As centerpoint enter: 4.890443985625159, 52.37019700877677 (you can of course reproduce with any other position as well)
• As zoom level enter: 12 (you can choose another level, but higher levels will make the bug more obvious)
• Click "Save and edit".
• If you enabled preview of the map earlier, you'll notice, that the map at the top is now centered on the host city of the next DrupalCon in Europe.
• Switch to the "Center & bounds" tab again. You'll see that the "center help map" is not centered at the same position as the preview map at the top. The form value of the centerpoint field is now 4.921874991931841, 52.26815737142193, which is wrong.
• Click "Save and edit" again. Your map displayed at the top is now centered on the wrong center point.

Discussion

The mapformvalues behavior is responsible for setting the correct center and zoom for the help map and applying any updates made on the map to the form. The problem lies with the zoomTo method used there. It somehow slightly changes the position during zoom (tested in OpenLayers 2.13.1) and the form value is updated with the wrong position. zoomTo has a second parameter to provide a LonLat object as the source of the zoom. If you do this, the map remains centered at the correct position. However, according to OpenLayers source code, this parameter is not part of the official OpenLayers API. The setCenter method allows adding a second parameter for the zoom level, which is part of the official API.

The downside of using setCenter is, that setCenter will not trigger the update of the form, since the map is not animated during zoom like with zoomTo and thus the moveend event is not triggered. I don't think, this is a problem though, as openlayers.js provided by the OpenLayers module also uses setCenter to set initial center and zoom and the value is probably converted to the correct projection beforehand like it is in the behavior – although I didn't check that. I also think, that it is a good idea to use the same method for setting center and zoom in both cases.