Code review comment for lp://staging/~michihenning/thumbnailer/vs-thumb-refactor

> It looks like any output to stderr from vs-thumb will be discarded on a
> successful thumbnail. This is probably okay, but might be a bit annoying if
> following the logs while trying to determine why vs-thumb is stuck somewhere.

Good point, thanks! I've changed it to leave stderr alone, so anything written by vs-thumb ends up going to the log file immediately.

I renamed data() to read() and added an assertion that triggers if we call it a second time.

For the .tiff extension, I changed this after I noticed that we still had a bunch of tests that incorrectly used a .jpg extension. In turn, that caused problems when I looked at vs-thumb output with a browser (or image viewer of some kind, I can't remember the details). Also, when testing, I quite often run vs-thumb manually. So, I added this mainly as a convenience. I don't think it does any harm?