packing engine now independent of numcols

This is a first.  The packing/constraining engine is now totally
independent of the number of columns and depends only on the number of
boxes.  The pre-simplex renderer could never do this.

The following template renders just as fast as if you divide all the
col/colspan's by 1,000:

  <ui:box fill="blue" id="blue"                       cols="5000">
      <ui:box width="10" id="a" fill="gray"           colspan="1000"/>
      <ui:box height="10" fill="white" maxwidth="100" colspan="1000"/>
      <ui:box height="10" fill="black" minwidth="100" colspan="2000"/>
      <ui:box width="10" id="b" fill="gray"           colspan="1000"/>
  </ui:box>

  - 

-- 
"It's lucky," he added, after a pause, "that there are such a lot of
islands in the world.  I almost envy you, Mr. Watson."

                                                   -- Mustapha Mond