ImageBuffer b(cx, yy - y);
	RGBA* t = b;
	int y0 = y; // <<<!
	while(y < yy) {
		if(progress(y - y0, yy - y0)) // <<<!
			return Null;
		memcpy(t, ~GetLine(y) + x, cx * sizeof(RGBA));
		t += cx;
		y++;
	}