Slicer in volume view

[ah548455]

By DGB:
Slicer in volume view: voxel dimensions should be used to scale the slice maps. E.g.: looks really odd. Voxel dimensions entered on upload page are used to scale axes for the shader, should also for the slicer. And suggest slicer have a background opaque rectangle, not black, but e.g light gray, to obscure background shaded volume.