print error and 4.0.1

[Michael Godfrey]

Rik,

I thought about the suggestion of enclosing the actual print call to OpenGL
in visible, off ... visible, on...  When I considered this it seemed like
overkill. But, now I wonder: is there any reason to have visible on while
the plot is being printed? If not, this could not only be a helpful fix
for 4.0.1, but it stay in for as long as it takes... It would still be nice to
resolve the seg fault bug, but it is likely from what I have learned that
it is not entirely an Octave problem. I think that the bug report to
the OpenGL folks is still open.

The full test should be:

if (visible on}
    visible off
    actual print xxx
    visible on
else
    actual print xxx
endif

Michael