Implement display of roots more faithfully
This is a follow-up to
758de957.
- unify the metrics and drawing of \sqrt and \root using helper
functions mathed_root_metrics and mathed_draw_root.
- compute the vertical spacing above the nucleus of the root following
rule 11 of the TeXbook. In particular, it is different in inline and
display style.
- draw the root glyph without hard-coded pixel values. Make the line
width depend on the zoom.
more work is needed to implement properly rule 11:
- Ideally, we should use sqrt glyphs from the math fonts. Note that
then we would get rule thickness from there.
- The positioning of the root MathData is arbitrary. It should
follow the definition of \root...\of... in The Texbook in
Apprendix B page 360.
Fixes bug #10814.