[css-writing-modes] text-combine-horizontal combining sequences

I've just updated the definition of 'text-combine-horizontal'
to match the discussions at the Paris F2F wrt inheritance etc.

As part of this, I've added a section on sequence determination:
   http://dev.w3.org/csswg/css-writing-modes/#text-combine-sequence

  # To avoid complexity in the rendering and layout, text-combine-horizontal
  # only combines consecutive characters that are not interrupted by an
  # element boundary. Therefore the UA must evaluate for combination only
  # uninterrupted runs of text.
  #
  # However, to avoid combining only part of a sequence, if the boundary
  # of a run is due only to one or more inline element boundaries, the
  # UA must inspect any characters that appear immediately before and
  # immediately after the run. If these characters would, without the
  # intervening element, form a sequence that could (if it were not too
  # long) combine, then the candidate run does not combine.

The second part is to solve the case of

   tcy { text-combine-horizontal: digits; }

   <tcy>Some text <span>12</span><span>34</span></tcy>

In this case, there's a 4-digit number. It shouldn't combine,
because the maximum is 2.

But because there happens to be an element boundary there,
the UA will see it as two numbers of 2 digits each. (This
is a problem we noticed while drawing up examples for the
discussion at the F2F.)

So the paragraph there tries to handle this case. It's
marked at-risk for now.

~fantasai