Log Message:
Changes 10.18.13:
* ppmtompeg: fix crash with input that is not a multiple of 16 pixels high
  and wide
* pamtotga: fix crash due to freeing nonexistent color table
* pamtotga: fix bug: invalid TGA when you use -rgb with a non-color input
* pgmtopbm: fix calculations that divide by maxval+1; should divide by maxval
* pbm_readpbmrowpacked(): Fix bug: ORs bits into return value instead of
  setting them