Linux workaround

Now that I was able to work on the Linux crashes, I immediately found
that the crash occurs on the following statement in the realize
routine in src/core/display_kernel.cpp:

// The test is a hack so that subclasses not bothering to implement
getProcAddress just
		//   don't get any extensions.
		if (getProcAddress("display_kernel::getProcAddress") != notImplemented)
			glext.init( *this );

If this statement is commented out, VPython runs on the latest Ubuntu,
but there are no materials. I'll look deeper into this, but if you're
capable of building from source you can have a working VPython just by
compiling without the offending statement. The comment "The test is a
hack" suggests something that might well go bad with a system change,
though this hasn't been a problem on Windows or Mac.

Bruce Sherwood

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