Add support for reporting application errors.

Created by: kbrenneman

This adds a new environment variable, __GLVND_APP_ERROR_CHECKING, which will tell libglvnd to act in a sort of strict mode when it comes to application errors.

Since current pre-glvnd drivers can often cope with a lot of app errors, libglvnd has to be similarly tolerant by default. This adds a way for an application developer to take advantage of stricter error behavior during development. Or more likely, it's something that an SDK could enable for debug builds.

In any case, the error that this commit checks for specifically is trying to call an OpenGL function without a current context. So far, all the broken applications that I've seen (and that the developers refuse to fix) is some variation on that.