ffplay: use a separate struct for the rescaled YUVA AVSubtitle rectangles

Current code segfaults since the deprecation of AVSubtitleRect.pict because it
freed/realloced AVSubtitleRect.pict.data by itself.

The new code stores the generated YUVA AVSubtitle rectangles in their own
struct and keeps the original AVSubtitle structure untouched, because
overwriting it is considered invalid API usage.

Signed-off-by: Marton Balint <cus@passwd.hu>
