Related fdo#71616: Set correct BaseSize

BaseSize has to be set in 100th mm, otherwise the scalable
brackets are far too thin.

See starmath/source/cfgitem.cxx:

pFormat->SetBaseSize( Size(0, SmPtsTo100th_mm( nTmp16 )) );

nTmp16 is 12pt -> 422 100th mm (at least for me). Let's reuse that.

Change-Id: I69571ae002f790199ab423955b1635eda42096eb
Reviewed-on: https://gerrit.libreoffice.org/9524
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>