libhb: fix merging of ssa subtitles

mergeSubtitle was originally written for pure text subs.  But now the
subtitle buffers have extra ssa formatting that needs different treatment.


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6374 b64f7644-9d1e-0410-96f1-a4d463321fa5