Always lowercase filenames in CUtlFilenameSymbolTable to fix loading packed files on Linux (fixes #865)

[rtldg]

Description

Tracked down in #865, loading packed files (e.g. from bsps & vpks) is often broken on community maps for Linux users (& servers!!!) leading to missing textures and broken model collisions.

Also reported at ValveSoftware/Source-1-Games#6868

I'm making this PR to bring more visibility to the cause and to hopefully be merged.
(The cause being that CUtlFilenameSymbolTable only lowercases filenames on Windows, which is called from CZipPackFile::GetFileInfo() to use a cached file-handle.)

[Tolfx]

Have you compiled and tested it works on all platforms like normal? As well fixing the issue ValveSoftware/Source-1-Games#6868?

[rtldg]

Have you compiled and tested it works on all platforms like normal? As well fixing the issue ValveSoftware/Source-1-Games#6868?

It's used in engine code (CZipPackFile) which we can't rebuild, so it can't be tested by compiling, but it has been confirmed to fix ValveSoftware/Source-1-Games#6868 after hooking the relevant functions:

• client-side Possible case folding on material loading results in missing textures Source-1-Games#6868 (comment)
• server-side https://github.com/rtldg/linux_file_lowercaser