This also changes behaviour a bit, previously we interleaved column and column group backgrounds. where we now put all the column group backgrounds behind all columns.
I believe this is the correct ordering as per CSS2.2 Appendix E.
Column backgrounds can overlap when using 'span', and we now render this in a different order, but this matches what other browsers do.
Differential Revision: https://phabricator.services.mozilla.com/D29278
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extra : moz-landing-system : lando
Matt Woodrow