sub: avoid unneeded calls to is_animated

4e5d996c3a3f74a7308aee53614c2b94c71daebc added this as part of a series
of patches written to avoid wasteful sub redraws when playing a still
image with subs. The is_animated special case was specifically for ASS
subtitles that have animations/effects and would need repeated redraws
in the still image case. This check was done unconditionally for all ASS
subtitles, but for very big ASS subtitles, this text parsing can get a
bit expensive.

Because this function call is only ever needed for the weird edge case
of ASS subtitles over a still image, some additional logic can be added
to avoid calling is_animated in the vast majority of cases. The animated
field in demux_packet can be changed to a tristate instead where -1
indicates "unknown" (the default state). In update_subtitle, we can look
at the current state of the video tracks and decide whether or not it is
neccesary to perform is_animated and pass that knowledge to sd_ass
before any subtitle packets are decoded and thus save us from doing this
potentially expensive call.