(#605): Compute 'bolder' and 'lighter' font-weight correctly

Previously, font-weight="bolder" or "lighter" would map unconditionally to pango's Ultrabold and Light, which is wrong as those are relative values that must be computed with respect to the inherited value.

We now do this:

Note that this is different from https://www.w3.org/TR/2008/REC-CSS2-20080411/fonts.html#propdef-font-weight which simply made "bolder" and "lighter" go up and down the 100..900 scale. That had the effect of not always performing a visible change in weight, since most font families only have a few weights available and do not cover the whole range.

Fixes #605 (closed)

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