TODO
- What about making a Windows installer?
- What about making a small GUI for windows?  If run, open a file browser, then convert and display
  result in notepad, or internet explorer.
- About process_scanline, it might be much better (clearer, faster) to pass a function pointer to the
  read pixels algorithm, so we don't have to do all those "if"s in the innerloop.  Might also be slower, though,
  because of the increased stack pushing and popping.
- Check j2pa --color ~/tmp/cola.jpg, should not get blue color there, because we are using grayscale colors!
- Consider using Bresenham scaling (search for "Image scanling with Bresenham" on DDJ.com)
- Consider using other libs for image conversions, e.g. would be nice to read GIF.  (See gdlib.)

BUGS
- Floating-point calculation (and rounding) is not 100% equal from compiler to compiler.  Try to fix this.