Make reference-image comparisons and updates work
Add and use copy_to_log_dir function
Change output filename format

This tries to group together related output files in a directory
listing, with the file-path disambiguation stuff moved to the end.
Get rid of temporary variable
Throw away error output in images_differ function
When a reference-image compare fails, give more info
Support PRELOADS and POSTLOADS files

In a test directory, these can contain lists of additional scene files
to load before/after each test file.
Really cleanup temp files in cleanup_cmd
When a PBRT output compare fails, give more info
Make image-comparison threshold configurable
Always use $ref_ext for reference images