An important @page declaration loses to a plain one in a :first, :left, :right or named page rule #640

With cssMode standard, #638 honours !important. For @page rules, it only does so between declarations of the same selector.

An important declaration in a plain @page rule still loses to a plain declaration of the same property in a @page :first, :left, :right or named page rule:

@page { margin-top: 30mm !important; }
@page :first { margin-top: 10mm; }

The first page gets 10mm. In CSS Paged Media it gets 30mm.

Mpdf::SetPagedMediaCSS() merges those rules one over the other, and does not look at importance. A fix would re-apply the important declarations of every @page key it merged, after the merge. The even-page margin swap has to be kept.

mPDF's @page ordering already differs from CSS Paged Media in other ways, so this is left for when someone needs it.

Found while working on #638. Part of #532, part of #520.