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)