This enum used to store very precise state about the decoding process,
let's simplify that by only including two steps: HeaderDecoder and
BitmapDecoded.
The `read_image_data` function of each one of[PBM, PGM, PPM]Loader use
the same structure to read an image. This patch harmonizes the three
functions and use finite loops instead of reading until EOF. It allows
to quit early on bloated file, but it's mainly done for refactoring
purpose.
