title: In a TIFF create a Sub IFD with thumbnail (libtiff)
body: <p>I know thumbnail.c includes some code that creates a thumbnail and places it in a sub IDF, but there is a lot going on in that code (generating the thumbnail, applying a contrast curve, etc.) and I am having difficulty reproducing just writing a thumbnail. Google has not been any help either.</p>
  
  <p>My question is, after I have opened an output file and have a TIFF*, I have my thumbnail data all ready to go (as well as my main image data), how do I add them in such a way that the thumbnail is in a sub IFD of the main image IFD?</p>