Draw used markers ontop of unused markers

[Chau]

Currently it looks like markers are drawn to the screen in no special order.

It would be nice if the unused markers were drawn first and then the used markers ontop of them. Then it would be easier to see how the used markers are distributed in the photo.

This is relevant both in the model view and in the photo view.

[Chau]

In model view it is possible to switch on/off Cameras, Markers, Region, Trackball and Info.

It would be nice to be able to distinguish between used and unused markers, so it would be possible to hide unused markers and only show the used markers.

This would make it easier to see the distribution of used markers in the entire model.