luxrays: rename GetConfigDir as GetCacheDir and use XDG_CACHE_HOME according to specifications

[illwieckz]

Rename GetConfigDir as GetCacheDir and use XDG_CACHE_HOME according to specifications.

• ~/.config should only be used as a fallback if XDG_CONFIG_HOME isn't set.
• ~/.config or XDG_CONFIG_HOME is a config directory, not a cache directory.
• the cache directory is XDG_CACHE_HOME, is not set by default and should fallback on ~/.cache.
• getenv returns nullptr but boost::filesystem::path doesn't accept nullptr.

This work is courtesy of I ♥ Compute initiative funded by 🇫🇷️ rebatir.fr.

[CodeFHD]

I had a look after this was mentioned in #662

Looks OK to me.

• Affects only Linux
• Uses an environment variable only if defined
• uses .cache instead of .config, which sounds resonable

No idea if there was an original reason not to use .cache, the commit history only mentions the use of /home/ vs /tmp/ to avoid automatic deletion of the caches.

If merged, I suggest to adapt the rest of the function for consistency, I.e. return kernelCacheDir instead of kernelConfigDir. The same instead of the variable name xdgCacheHome used by this PR, as the variable is not necessarily originating from XDG.

[howetuft]

I'll take care of that soon, the destination should be version 2.11

[howetuft]

Just a point of verification, since I see that this proposal is made on courtesy of a commercial company:

LuxCore is licensed under the Apache 2.0 license, so all contributions are made under this license. Before I merge, @illwieckz, can you please confirm that you explicitly agree with that?

[illwieckz]

Just a point of verification, since I see that this proposal is made on courtesy of a commercial company:

LuxCore is licensed under the Apache 2.0 license, so all contributions are made under this license. Before I merge, @illwieckz, can you please confirm that you explicitly agree with that?

Yes, it is explicitly permitted to merge this contribution under the Apache 2.0 license.

[illwieckz]

And just in case:

I explicitly permit to merge this contribution under any license.

[howetuft]

OK, thank you! I made just a few adjustments as I switch from boost::filesystem to std::filesystem in v2.11