doc: loaders: Add PSD + cleanup.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
PCX: Implement PCX RLE I/O stream.

The loaders now internally uses RLE I/O stream.

TODO: Check errors from GP_IORead() in inner loops
      More tests

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