starmath: no need to set those on every paint event

Change-Id: Ife149baf15d6f38a2a316e124b78bdb7e1b64967
starmath: directly create utf8 OUString

This fixes:

warn:rtl.string:14867:1:sal/rtl/strtmpl.cxx:1269: rtl_uString_newFromLiteral - Found char > 127

Change-Id: I1f868406501c83918fc3c06dd0b68eb1f6604f26
Related fdo#71616: Fix scrolling

Change-Id: I9c57de1179d7db19f1fa2b104017dd379f854749
starmath: remove hard-coded value

Change-Id: I58dad585c5de24ebd06ce9bcc64e49e8973850bb