3D box with arbitrary vertices (cuboid).
Vertices 0-3 specify the "bottom" rectangle in clockwise direction and vertices 4-7 the "top" rectangle so that 4 is above 0, 5 above 1 and so on.
If vertices are provided some local coordinates the transformation matrix of the element should also be set (but then the memory usage is increased by the size of the REveTrans object).
Currently only supports 3D -> 2D projections.