Prevent nemo trying to generate a thumbnail more than 5 times before
giving up, instead of getting stuck in an endless loop.  The thumbnail will
still try to regenerate when the file is replaced or nemo is restarted.

This is a workaround until the root cause of this issue is found.. possibly
upstream in gnome-desktop
Reduce thumbnail tries to 1
Add incrementer to nemo-thumbnails.c
Merge pull request #350 from mtwebster/thumbnail-fix

Prevent nemo trying to generate a thumbnail more than 5 times before