added subtitle settings to set ASS/SSA subtitles position

Many users want to show ass/ssa subtitles on the black screen at the bottom of the video. So add a option to set the location of ASS/SSA subtitles on the screen:
In the video: ASS/SSA subtitles will show the position fit the video
In the screen: ASS/SSA subtitles will show the position fit the screen (use the bottom black bar)