RFC: thumbnail: Modify to skip unreadable files

[Dr. Michael J. Chudobiak]

Hi all,

Another patch to consider for the thumbnail spec, based on a discussion 
4 years ago [1].

Basically, thumbnailers should not generated "failed" thumbnails for 
unreadable files, or they will remain un-thumbnail-able even if their 
permission are changed to make them readable.

[1] http://www.mail-archive.com/xdg@lists.freedesktop.org/msg04016.html
[2] https://bugs.freedesktop.org/show_bug.cgi?id=49799

Comments?

- Mike