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.
Also,  fixes a bug: when a new axis position was selected, the axis were not updated until the mouse moved.