subtitles size shouldn't depend on the resolution of the video

Submitted by Guillaume Desmottes @gdesmott

Link to original bug (#370715)

Description

I configure the size of subtitles in totem so i can watch them comfortably.
It works fine with most of my videos (with external .srt file) but with high resolutions ones (like 1280x720) subtitles are too small.
I think the size of subtitles should not be dependent of the resolution of the video played.