add tool to manipulate fade intensity in depth cueing and contour penwidth

[janathbdesign]

The tool to set the fading appears to be lineair based while from an architects point of view there is the need to tweak the fade differently. One might want to fade elements more then an algorithm would determine based on a distance from the viewplane. Also, contours that are further back should not have the same penwidth if possible. would this be possible?