Make opj_set_decode_area() and opj_decode() take into account opj_set_decoded_resolut...

Make opj_set_decode_area() and opj_decode() take into account opj_set_decoded_resolution_factor() (#1006, affect API use)

* Better document usage of opj_set_decode_area(), ie expecting coordinates
  in full resolution/reference grid even if requesting at a lower resolution
  factor
* Make sure that image->comps[].factor is set by opj_set_decode_area() and
  opj_decode() from the value specified in opj_set_decoded_resolution_factor()
* opj_decompress: add 2 environmenet variables to test alternate ways of
  using the API, namely USE_OPJ_SET_DECODED_RESOLUTION_FACTOR=YES to use
  opj_set_decoded_resolution_factor() instead of parameters.cp_reduce, and
  SKIP_OPJ_SET_DECODE_AREA=YES to not call opj_set_decode_area() if -d is
  not specified.