libass: use the visible area dimensions, not the input dimensions
libass: use the visible area dimensions, not the input dimensions

Fixes incorrect offset when playing http://streams.videolan.org/misc/test-files-ccc/vsfilter_general_TestCase.mkv

Signed-off-by: Jean-Baptiste Kempf <7b85a41a628204b76aba4326273a3ccc74bd009a@videolan.org>

shyamalschandra/vlc,xkfz007/vlc,shyamalschandra/vlc,krichter722/vlc,shyamalschandra/vlc,krichter722/vlc,vlc-mirror/vlc,shyamalschandra/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,vlc-mirror/vlc,shyamalschandra/vlc,krichter722/vlc,krichter722/vlc,shyamalschandra/vlc,krichter722/vlc,xkfz007/vlc,shyamalschandra/vlc,vlc-mirror/vlc,vlc-mirror/vlc,vlc-mirror/vlc,xkfz007/vlc,vlc-mirror/vlc,xkfz007/vlc,vlc-mirror/vlc,krichter722/vlc,krichter722/vlc