message: Check for GL debug extension when using it
  Apple does not support glDebugMessageCallback
diff: @@ -1147,7 +1147,10 @@ void GLReplay::DeleteDebugData()
  MakeCurrentReplayContext(&m_ReplayCtx);
  + if(HasExt[KHR_debug])
  + {
  GL.glDebugMessageCallback(NULL, NULL);
  + }
  if(DebugData.overlayProg)
  drv.glDeleteProgram(DebugData.overlayProg);
  @@ -1163,7 +1166,10 @@ void GLReplay::DeleteDebugData()
  MakeCurrentReplayContext(m_DebugCtx);
  + if(HasExt[KHR_debug])
  + {
  GL.glDebugMessageCallback(NULL, NULL);
  + }
  ClearPostVSCache();
  