Add process for programs to voluntarily remove renderdoc. Refs #115

* To remove renderdoc (e.g. if the API version doesn't match what is
  expected), user code can call RENDERDOC_Shutdown() to remove the running
  thread and undo any library hooks, then unload the dynamic library.
* This still isn't recommended as there could be unexpected side-effects,
  and it will definitely break if this happens after any APIs have been
  initialised, but it's an option.