aliased & packed?

[Tom Moran <tmoran@bix.com>]

Clearly you can't do it.  I'd like to use pragma Pack for bits in a
black/white bitmap array.  That's easy to manipulate.  But to actually
display it I need to do a system call and pass the address.  If it
wasn't packed, Image'unchecked_access would be appropriate.  But
that won't work because Image would have to be aliased, and it can't
be because it's packed. Must I drop 'Pack' and make it an array of
bytes and do my own masking, shifting, etc?  What's the cleanest/most
portable/best way to accomplish the task?