Eliminate marker_offset parameter

[danepowell]

The problem is that the controls for markers aren't always centered on the actual marker position. The marker feedback topic only provides marker position, so if the controls are too far away, we won't be able to 'grab' them.

To get rid of this parameter, we'd need to capture server state (via the update topic), find the marker corresponding to that in the feedback topic, find the best candidate control (MOVE_ROTATE_3D), and then set the marker position to this control position. Ugh.