Remove gap in middle of axis, and set cursor position to axis position on view exit

Quick patch following a request by a user: we no longer have a gap in the center of the axis, it was making proper aiming on a voxel too hard, and if one wants to see what's around, one can always zoom, axis width is always 1 screen pixel, so it's often thinner than a voxel. Also, cursor position is reset to center axis position when the view is exited by the mouse, to allow for comparison between multiple linked views of the selected voxel's value etc.