agsprite: preliminary support to create acsprset from extr. pics

agsprite: preliminary support to create acsprset from extr. pics

after extensive testing with Quest for Infamy, code works as
expected. currently there are some issues:

- ags doesn't automatically detect that sprindex.dat is bogus
  it needs to be removed when agspack'ing the files together
  (edit agspack.info and comment line containing sprindex.dat
   with a leading # character, and edit filecount to be one less).
  future version will probably add a command line option to
  create a new sprindex.dat which can be used to replace the original.

- on the start screen of QFI, the blinking shield sprite isn't displayed.
- on the intro of QFI, cut scene with hay cart isn't visible
- in Last 'n Furious, menu selection underlay bar isn't visible

the latter 3 issues may be related to how AGS treats alpha channel on
32bit images.

future versions of agsprite might support an option to reduce all
32bit images to 16bit (hi-color), as this could cut game size in half
while having hardly any visual difference.

addressing #4