I think sdl2:rwops created from CHICKEN-managed memory are not
stable, so to be safe the data should be evicted using object-evict.

The u8vector data is now converted to a blob, because evicted
u8vectors cannot be used. The u8vector procedures do not recognize
them as u8vectors.

Also fixed the tongue image, which was accidentally saved in a format
that SDL cannot load. It was 32-bit, with an alpha channel.

Also added the three original BMP files to the demos directory:
face-frown.bmp, face-smile.bmp, and face-tongue.bmp.