Loaders: GP_IOFill: Set errno on end of file.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
loaders: Initial PCX support + tests + doc.

TODO: Needs a buffered IO Stream

FIXME: More tests, (256 color palette)
       does v2.5 with filed palette work?

Signed-off-by: Cyril Hrubis <metan@ucw.cz>