replace StringBuffer with StringBuilder

[rimuln]

Years ago was in our company made some internal fork based on Batik 1.8. We want switch back to OSS official version. It was back-ported some changes from newers Batik to our 1.8 to be able use JDK9+ but from overall we want use official OSS in future. As original developer isn't in our company some years we don't know what all was done. He committed state after changes not the original state at first:( We know only that we had performance and memory problems.

We tried to compare official 1.8 with our 1.8.

One probably usefull change was this migration from StringBuffer to StringBuilder.