fix problem with spurious timestamp change

fix problem with spurious timestamp change
some rearrangement of code that was previously done to reader caused
scr_offset to be subtracted from renderOffset twice whenever a
new scr_offset was calculated.  this could cause subsequent timestamp
calculations to be way off and in at least one known case lead to
a crash due to consuming too much memory in hb_buffer_t's

git-svn-id: svn://localhost/HandBrake/trunk@3399 b64f7644-9d1e-0410-96f1-a4d463321fa5