filesystem: SDL_GetUserFolder() now follows the SDL_GetStringRule.

It also now caches at the higher level, so the platform-specific bits don't
change their interface much.

Reference Issue #10229.

src/filesystem/unix/SDL_sysfilesystem.c

@@ -510,7 +510,7 @@ static char *xdg_user_dir_lookup (const char *type)
     return NULL;
 }
 
-char *SDL_GetUserFolder(SDL_Folder folder)
+char *SDL_SYS_GetUserFolder(SDL_Folder folder)
 {
     const char *param = NULL;
     char *retval;