Stylesheet rules that name sup or sub never apply #649

A stylesheet rule whose selector names sup or sub never applies, in either cssMode. A class rule on the same element does apply:

<style>sup { color: #f00 }</style>
<p>x<sup>2</sup></p>        <!-- the 2 is drawn black -->

<style>.s { color: #f00 }</style>
<p>x<sup class="s">2</sup></p>   <!-- the 2 is drawn red -->

SUP and SUB are missing from allowedCSStags (src/Config/ConfigVariables.php), so rules keyed by those tags are never merged into them. That means common resets such as normalize.css's sub, sup { font-size: 75%; line-height: 0; position: relative; vertical-align: baseline } do nothing.

mPDF draws sup and sub with its own size and offset (textvar). Adding the tags to the list has to be checked against that, so that a rule changing font-size or vertical-align takes over from the built-in handling rather than stacking on it. Check whether any other inline tag mPDF supports is missing from the list too.

Reproduced on gravitypdf at e685435. Found while working on #646. Part of #552.