libass doesn't free any font data it requests until the ass_renderer is
destroyed, so recreate it every time the font is changed to avoid
holding on to extremely large amounts of data when the user quickly
switches between fonts.
Closes#1320.
At least on OS X, allocating the buffers is one of the more expensive
parts of video playback, and on an arbitrary 720p H.264 file with simple
subtitles this cuts CPU usage while playing by about 30%.