EMI: Fix display of movie subtitles

This PR fixes three issues with the recently added feature to display subtitles in EMI movies:
 * Subtitles were not properly centered because the width calculated by Font::getStringLength was always too big. The function was changed in c69322d7, but I don't really understand the reason behind it.
 * When the movie playback was stopped with Escape the last displayed subtitle would remain visible.
 * Subtitles were not rendered with TinyGL because EMI dims the screen during movie playback.