When 3DLDF is run, there is only one thread of execution.
However, it could benefit from the use of multiple threads.
In particular, it may be faster and more efficient to have
Picture::output() run in its own thread.
In this case, it will no longer be possible to share
current_picture among figures.
It may also be
worthwhile to execute the code for "figures", i.e., the code
between beginfig() and endfig(), inclusive,
in their own threads. This will require some changes in the way data
are handled. For example, if non-constant objects are shared
among figures, there may be no advantage to multi-threading because of
the need to coordinate the access of the threads to the objects.
If threads are used, then non-constant objects should be
declared locally within the figure. They may be locally declared
copies of global objects. Alternatively, beginfig() could be
changed so that objects could be passed to it as arguments, perhaps as
a vector<void*> and/or a vector<Shape*>.