wgl: Do not reach out and destroy contexts on cleanup.

wgl: Do not reach out and destroy contexts on cleanup.

Simply skip cleanup when contexts are still active.

This addresses two issues:

- in some situations the ICD DLL may be unloaded before the DLL that is
  using GL contexts is, so we may receive GL calls after stw_cleanup.

- when aborting (exception, or control-c) the contexts may have been left
  in an inconsistent state and attempting to destroy can cause
  unpredictable results.