[css-shapes] Redefine shape-margin

While I think the definition of shape-margin is pretty intuitive, I don't
think it's rigorous enough for use in odd corner cases. It relies on the
definition of "perpendicular" (which doesn't work for corners, so you have
to add a special case), and also on a direction "outward" from the shape,
which is awkward for cases of self-intersection.

I suggest something like the following:
"""
The shape-margin property adds a margin to a shape-outside. This defines a
new shape, defined as the path enclosing the smallest area that includes
all the points that are closer than the shape-margin distance from a point
inside the underlying shape. This property takes only non-negative values.
"""

This relies only on the concept of distance and "inside" the shape, which
is well-defined (and may depend on the value of the fill-rule property). It
could then be followed with an explanatory note with something similar to
the current wording (including the shrink-wrap bit, I think that's a great
analogy!), as that's probably simpler to understand and would apply to 99%
of cases anyway.

I realise this definition is a bit harder to follow (although I'm sure the
wording I've suggested can be improved) but I think it's important we get
this right now so we can re-use the definition in later levels.

Technical notes:

   - There's no need to specify points closer than *or equal to* the
   shape-margin distance; the path which encloses all such points must include
   their limit anyway. Whether it's clearer or not to include that wording is
   another matter. :-)
   - We could just say "points that are closer than the shape-margin
   distance *from the underlying shape*", as the usual definition of
   distance from a shape is the infimum of the distance from the points in it
   anyway, but I think it's actually clearer to spell it out like this.
   - This definition clarifies the status of infinitely thin spurs: since
   they do no enclose any points, they do not contribute to the margin.


Thanks,
Tom