SdStyleSheetPool::GetBulletFont can be static

...that's why the problem fixed in 0466c0dcddd3b5af02c711794e8cd4c3c6c12993 "Set
mxStyleSheetPool before using it" had never caused trouble in practice (and its
change to SdDrawDocument ctor can be reverted again)

Change-Id: I662995ab4fcaa7ae461cb9575666825d8a869735