The libglvnd libraries will now check for a new environment variable,
__GLVND_APP_ERROR_CHECKING. If it's set to a non-zero value, then libglvnd will
check for and report some application errors.

Many non-libglvnd implementations of libGL.so have been fairly tolerant of
certain application bugs. Libglvnd has to be similarly tolerant to support
existing apps that the developers can't or won't fix.

The new __GLVND_APP_ERROR_CHECKING provides a way to check for some of those
errors, so that hopefully there will be fewer broken apps in the future.

In addition to the setting itself, this updates the no-op stubs in
libGLdispatch to report an error when the app tries to call an OpenGL function
without a current context. Later changes will likely add other error checks.