[css3-page] Replace 6.3.2 by a new, simpler algorithm.

This algorithm sizes page-margin boxes in a series of conditional steps
rather than by optimizing a quadratic function under constraints.

https://www.w3.org/Style/CSS/Tracker/issues/255

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extra : rebase_source : dc941bde65c150cf8a621fd341f322af2dd8d9e8

Author: SimonSapin