Implement .pic to .2bpp decompression

[Rangi42]

Fixes #350

This correctly uncompresses every .pic file to exactly reproduce its .2bpp data:

$ for f in `find gfx -name '*.pic'`; do tools/pkmncompress -u $f ${f%pic}tmp; done
$ for f in `find gfx -name '*.tmp'`; do cmp $f ${f%tmp}2bpp; done
$ find gfx -name '*.tmp' -delete