Improve file naming with phpthumof.hash_thumbnail_names off

Fixes two problems:
1. Gives different names to files in different directories which happen to have the same name, phpthumb options and be called from the same resource.
2. Uses the same name when a phpthumbof is called from different resources on the same file with the same options.

See more discussion of the issue [here](https://github.com/splittingred/phpThumbOf/issues/48).