Debug levels of logging messages printed by Vortex.

Used with VortexLogHandler, a callback which lets the client print logging messages itself.

Enumerator:
VORTEX_LEVEL_DEBUG  A debugging message; describes normal operation, not a problem.
VORTEX_LEVEL_WARNING  A warning message; describes a possible problem.
VORTEX_LEVEL_CRITICAL  A critical message; describes a definite problem.