[mathml] minsize/maxsize

This is implementing changes discussed in [1]:

- percentage minsize/maxsize should refer to the unstretched size,
  mo-minsize-maxsize-001.html is updated accordingly.

- minsize/maxsize adjustments are done by scaling the distance
  above and below the math axis proportionally.
  mo-axis-height-1.html is extended to verify calculations from
  [2]. These new formulas ensure that symmetry with respect to
  the math axis is preserved when minsize/maxsize is applied
  and two tests are added to verify that more directly.

- default value for minsize is 100% (the unstretched size) rather
  than 1em.

[1] https://github.com/w3c/mathml-core/issues/103
[2] https://w3c.github.io/mathml-core/#layout-of-operators

Bug: 332931380
Change-Id: I63ca77c5d0b8934570916d589160da0c90722f08
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5402478


Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: Frédéric Wang <fwang@igalia.com>
Cr-Commit-Position: refs/heads/main@{#1283539}