pnm file formats not accepting bitdepth greater than 8 bpp

Originally reported on Google Code with ID 7

```
Whatever the bitdepth specified in the header of pnm,pgm,ppm files, it will consider
255. 
Should change to read the specific bitdepth.

Functions affected :
* pnmtoimage
* imagetopnm


```

Reported by [detonin](https://github.com/detonin) on 2009-09-25 16:23:03
