GL drawing fixes, retain cycle fix, and other changes.

Hi!

I've been customizing the EZAudioGLPlot for my own purposes, and I've also made some changes that might be useful to contribute back. These are a handful of changes that I think are worth considering. I think they should be pretty self explanatory given the commit comments, but let me know if you have questions.

Thanks,
Andy

PS. I've also made other, much larger changes that I might open an issue for.