unified cache folder

[Wumpf]

There's plenty of data that wants caching for fast startup (and thus fast iteration!)
Should introduce a more intelligent/unified caching scheme that allows automatically removing outdated cache files.

Uses so far:

• compiled shaders
• signed distance fields
• hdr cubemap

should have a subfolder in a .cache folder for each of those. Instead of file hashing, just always rely on file modified date (good enough!). Evict old objects on cache miss