lavc: add support for CODEC_CAP_DELAY in subtitles

This patch adds CODEC_CAP_DELAY support to avcodec_decode_subtitle2.

For DVB teletext decoding, a single teletext packet can contain multiple
teletext pages. In order to support that, the teletext decoder may buffer
some pages.

Signed-off-by: Marton Balint <cus@passwd.hu>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>