IeResRef: added nice IsEmpty method and used to fix pst intro crash

pst has no tooltip bg
pst: fixed regression from 79b79b3155
Font: (partially?) fix SizeString

still seems there is a bug here as I can still trigger an assert.
Font: RenderLine can't know when it is safe to overrun

ideally this shouldnt happen anyway. currntly hacking around the only known situation with the portrait status icons.
TextContainer: fix errant newline conditional
Merge branch 'font-rewrite' of github.com:bradallred/gemrb into font-rewrite