Clear the global PGL on dispose().

A single-line patch.

As the comment says,

// This next line is critical to release many static allocations.
// This is important in the context of, say, a unit test suite, which
// runs more than one OpenGL sketch within the same classloader
// (as in the case of processing.py).

I never had this problem before, because only one of my processing.py unit tests had used an OpenGL renderer until recently.