libv4lconvert is not safe for using one convert instance as returned by
v4lconvert_create from multiple threads, if you want to use one v4lconvert
instance from multiple threads you must provide your own locking and make
sure no simultaneous calls are made.

libv4l1 and libv4l2 are safe for multithread use *under* *the* *following*
*conditions* :

* when using v4lx_fd_open, do not make any v4lx_ calls to the passed fd until
  v4lx_fd_open has completed

* all v4lx_ calls must be completed before calling v4lx_close
