Improve drawing of math roots (\sqrt and \root)

Improve drawing of math roots (\sqrt and \root)

This is preliminary work to make roots look more like in TeX output:

* correct font size for root order

* set minimum size of the root from the max ascent descent of the
  font. This is what TeX does.

* fix drawing, and in particular horizontal size.

More needs to be done, in particular:

* handle zooming and dpi correctly by removing hardcoded pixel values.

* factor the code for the two types of roots in one.