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.