Adapt scale of clip plane capping texture

[HuguesDelorme]

The scale of a clip plane's capping texture is fixed(it's 0.05, -0.05) So when the graphics objects displayed are small then the capping texture looks too "zoomed". In the opposite when the objects displayed are big then texture might not be visible at all. The capping texture scale should be adapted to the bounding box of the graphics objects within the scene. Also it might have to be adapted to the zoom factor of the view.

"Normal" case:

Texture looking to coarse:

Texture almost not visible:

FYI @VictorLamoine