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/dummy/SDL_sysfilesystem.c

@@ -37,7 +37,7 @@ char *SDL_GetPrefPath(const char *org, const char *app)
     return NULL;
 }
 
-char *SDL_GetUserFolder(SDL_Folder folder)
+char *SDL_SYS_GetUserFolder(SDL_Folder folder)
 {
     SDL_Unsupported();
     return NULL;