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>
(cherry picked from commit 3f106f42be8e7d5b66b74b0c6cf96a6f513eec90)
Signed-off-by: Jean-Baptiste Kempf <7b85a41a628204b76aba4326273a3ccc74bd009a@videolan.org>

jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2