This patch handles display specs within `scan_for_column', which mainly
works to handle it in `current-column' and `move-to-column', which then
effects the filling functions and commands, and visual-fill-column, and
so on.

The patch seems to be working, as far as I can tell, but it would be
nice if someone could take a look.  Specifically, I copied the code to
convert the Lisp_Object holding the width into an integer from the code
above handling the space display properties, so that needs
double-checking.

I decided to use `Fcar_safe' instead of `XCAR' when taking the `car' of
`Fimage_size' just to be safe but it doesn't look to be necessary?  If
that's fine, then feel free to make the change.