#6 Partial model-visibility

[Nicholas Bishop]

This means interactively selecting an area of the model
and hiding it. This makes it easier to work on a
portion of the model that would otherwise be obscured
by another part of the model. However, for the system
to be really useful, it must also make the visible
portion display faster.

An earlier attempt to add this feature failed because
of numerous bugs, but the basic idea was sound:
reorder the vertices and polygons in their respective
vectors. The specific order isn't important, as long as
all visible elements are grouped together at the
beginning, and all hidden elements grouped at the end.
Then, all operations which must iterate through these
containers, including the GL, need only include
[0,last_visible_index]