Fix an issue with keyframe positions produced by mythcommflag --rebuild.

The seeking code in DecoderBase assumes there is a keyframe at frame
0, otherwise it shifts all keyframe numbers by the value of the first
keyframe number in the map.  This causes problems for recordings with
variable keyframe distances.  To correct this, make sure mythcommflag
--rebuild creates an explicit entry for frame 0.

Note: this does not fix the issue of mythcommflag --rebuild adding
bogus keyframes to the seektable for h.264 recordings.

Cherry-picked from a small part of
49dbed5be0729b04a5f0fd0426a32fceb2dd7935 .
(cherry picked from commit ff4edc7e61e6f1f14161f67ab865b6e4d14ff80b)
Configure the h.264 parser to identify only IDR keyframes.

This makes keyframes identified by "mythcommflag --rebuild" and
mythtranscode much closer to reality, though there are still
discrepancies compared to the recorder and raw ffmpeg.
(cherry picked from commit 590f161b1d5ff8c7e08b0beae480ece7a185d3ab)
mythccextractor: Fix teletext subtitle duration

Previously ProcessTeletext() didn't leave one subtitle behind.  This caused the
length calculation in IngestSubtitle() to never be performed, which meant that
the length was always set to the default value 750 ms.

Fixes #11081.
(cherry picked from commit 08ad6cef707456e959b48d5e48c09929e023fc49)
(cherry picked from commit 018691f04687321f90ef2938455824065bcf674e)
mythccextractor: Skip teletext page headers
(cherry picked from commit 9519a103da6ad8dc589042fbda4fdfe4e1a2fca8)