cache rendered images to temp directory

[catdad]

• save files in tmp directory
• hash filepath to generate filename
• allow prefixes (thumbnail, raw, etc.)
• delete old files from tmp directory

[catdad]

It turns out that many RAW files are actually faster to read directly from the raw file (Sony, DNG) or at least just as fast (CR2), so caching these files is actually worse. I may change it up and only cache known slow files.

Also, I should check slow drives, like SD cards and whatnot. There may be a bigger benefit to caching there.