Various changes:

Various changes:
 - fix drawing text on 2/4 channel images for FT2
 - reading ASCII PBMs was broken, it assumed there was whitespace
   between samples but there doesn't need to be
 - add makemap type of mono/monochrome for producing monochrome images
 - roughly tripled speed of reading any sort of PNM
 - reading a pnm can now return a partial image if you set allow_partial
 - reading a bmp can now return a partial image if you set allow_partial
 - we can now read 16-bit/sample binary PGM/PPM images
 - we can now write 16-bit/sample binary PGM/PPM files if explicitly
   requested (since GIMP can't read them)
 - reading a tiff will now only return an incomplete image if you set
   allow_partial
 - some documentation reformatting