mpv saves cache by default nowadays, but vo_gpu is pretty spammy and
saves a bunch of files per shader. If someone is using the non-XDG
config directory, this all gets dumped directly into ~/.mpv which isn't
so nice. Save it to a sub directory called "cache" instead (or
alternatively submit to your XDG overlords). For unfortunate reasons,
macOS uses XDG_CONFIG_HOME and has the same legacy fallback mechanism,
so this applies to it too.
macOS really has completely different path conventions that mpv doesn't
take into account and it treats it just like any other old unix-like
system. This means mpv enforces certain conventions on it (like all the
XDG stuff) that doesn't really apply. Since we'd like to use more of
this but at the same time not distrupt mac users even more, let's just
copy and paste the current code to a new file, update the build and call
it a day. This way, the paths of these two platforms can more freely
diverge.
