fixed to use function ass_set_line_position 

1. exported function ass_set_line_position so xbmc can call it to set subtitles position use setting "subtitles.align".
SUBTITLE_ALIGN_BOTTOM_INSIDE
SUBTITLE_ALIGN_TOP_INSIDE: 
SUBTITLE_ALIGN_BOTTOM_OUTSIDE
SUBTITLE_ALIGN_TOP_OUTSIDE

2. fixed wrong clip. this bug let subtitle can't show when set line_position to top and set_use_margins as true.