SDL_GetPrefPath fails

[samsinsane]

SDL_GetPrefPath assumes that mkdir will create the intermediate directories but it doesn't.
The Unix version does this:

SDL2/src/filesystem/unix/SDL_sysfilesystem.c

Lines 230 to 237 in 33245dd

	for (ptr = retval+1; *ptr; ptr++) {
	if (*ptr == '/') {
	*ptr = '\0';
	if (mkdir(retval, 0700) != 0 && errno != EEXIST)
	goto error;
	*ptr = '/';
	}
	}

Perhaps this could be done in the Emscripten version too?