Log Message:
graphics/ximaging: update to version 1.8

# Changes (since last release)
* Support for external filters.
* Minor GUI enhancements ("Pass To..." menu command, SGI style path
  field, additional toolbar buttons, select by glob pattern).
* Added -f (force file type) command line option to the Viewer.
* Support for additional sub-formats in TGA and SGI readers.
* PNM/PAM image handling.