Stereoscopic subtitle offsets

This fixes some issues with subtitles/overlays when a stereoscopic depth have been configured.

It introduce a bug/sideeffect, but imho worth the tradeoff for now. After this pull dvd/bluray menu's will also get the offset applied if xbmc has been switched into a stereoscopic mode.

It should not be impossible to solve, but involves some changes that get's tricker to get in for gotham.