glretrace: Enable all debug output.

On nVidia systems, not all debug output is enabled by default. If we wish to
have the driver tell us where it is placing buffer objects, for example, we have
to enable all the debug channels by passing GL_DONT_CARE to everything in
glDebugMessageControlARB(...)
glretrace: Limit the low severity messages.

Otherwise some traces will be full of them...
glretrace: Fix typo.