Introduce simple cache memory management

Introduce simple cache memory management

libass' cache was basically unmanaged; it would grow without any limits,
depending on how complex the subtitles are.  Introduce a simple limiting
that resets the cache if certain limits are exceeded.  The bitmap cache
is limited to approx. 50 MB size by default, while the glyph cache
allows storing up to 1000 glyphs by default.  A few tests with rather
heavily softsubbed fansubs show that these limits are fine.
The API was extended with the function ass_set_cache_limits which allows
modification of these limits.