VideoCommon: Fix std::filesystem::path encoding conversion

[JosJuice]

In std::string, you can store strings using any encoding, but in Dolphin we have decided to use UTF-8. The problem is that if you convert between std::string and std::filesystem::path using the built-in methods, the standard library will make up its own assumption of what encoding you're using in the std::string. On most OSes this is UTF-8, but on Windows it's whatever the user's code page is.

What I believe is the C++ standard authors' intended solution to this is to use std::u8string instead of std::string, but that's a big hassle to move over to, because there's no convenient way to convert between std::string and std::u8string. Instead, in Dolphin, we have added helper functions that convert between std::string and std::filesystem::path in the manner we want. You always have to use these when converting between std::string and std::filesystem::path, otherwise we get these kinds of encoding problems that we've been having with custom textures.

Fixes https://bugs.dolphin-emu.org/issues/13328.

[iwubcode]

Thanks @JosJuice ! My original thought was to update Dolphin to actually take paths in places where it currently takes strings. This would be changing FileToString, GetTextureDirectoriesWithGameId, DoFileSearch, etc to use or return filesystem::path instead because that is cross-platform automatically. This avoids all these conversions.

However, this is admittedly simpler. I always forget about PathToString since it's not standard despite having used it a few times. I'll have to go update other places I am doing in current and future reviews.