ImageInput close/dtr leaving files open fixes
* For a couple formats, the ImageInput dtr didn't automatcially close() the
  file if it was still open.
* For a couple formats, a redundant call to close() might have strange
  effects because it didn't double check that the file was open.
Move shared_ptr and intrusive_ptr from the global to the OIIO namespace, to
avoid conflicts when another package already defines them.