Disable GL_DEPTH_TEST in PostProcessor

It took me a while to figure out why I was getting a black screen when I added PostProcessor to my app. GL_DEPTH_TEST turned out to be the culprit. 

I guess I should have disabled it before calling `postProcessor.render();`, but maybe having this call in the `render()` function might save some debugging time for somebody else... Let me know what you think.

By the way, awesome job with this lib :)