Rewrite get_delim_box()
The older version was always returning a box larger than requested and
with some fonts (e.g. STIX Math) extensible \overbrace was too much
wider than it should. I tried debugging it but it was too much complex
doing many clever things. The rewrite is partly stolen from XeTeX one,
further improved and is much simpler and had no regressions AFAICT.
This might fix #726 and other potential issues.
git-svn-id: https://foundry.supelec.fr/svn/luatex/trunk@4361 0b2b3880-5936-4365-a048-eb17d2e5a6bf