Don't open files to find max width for thumbnail

Hi,

We were opening files to find out the maximum possible width the thumbnail could be.  This is silly and unnecessary because it takes up too much memory if we want a lot of thumbnails at once (like the `reorder` screen does).  

1000 is a sensible maximum. Let's use that.

The unit tests are skipped because they rely on you having tiff utils.  You can run them manually if you like and they do work. ;-) 

Huw.