PCX import

[Martin Weber <martweb@xxxxxxx>]

I compared the time GIMP uses to load an image (several formats) into
memory (not display) with several other programs like ImageMagick, XV,
... For most formats GIMP has a very good solution, but for PCX
ImageMagick and XV loads the pictures 50 % faster into memory than GIMP.
Where could pcx.c be optimized?