Avoid un-necessary video mode changes.

Fixes #11494
(cherry picked from commit 8aca90207ffffc9dd512fa11cd9d88d1f4b5884f)
Revert "Avoid un-necessary video mode changes."

This reverts commit 3c763fdca88075f8ba7667f5096583d66307b2cd.
Avoid un-necessary video mode changes.

Fixes #11494
(cherry picked from commit 8aca90207ffffc9dd512fa11cd9d88d1f4b5884f)
minor code cleanup.

Following 8aca90207ffffc9dd512fa11cd9d88d1f4b5884f, remove useless test and tidy-up
(cherry picked from commit 4e23172d774cf36a45b7b7f382797a9887fa6de6)
Simplify mythtranscodes clean-cutting mechanism and fix a bug

This changes clean-cut to work as it was originally intended, to decode
the whole video, and perform cutting by discarding video and audio, rather
than seeking. Previous to this change, a combination of seeking and
discarding was used. Also fix bug where a frame within a cut area might
be kept as part of maintaining audio and video synchronisation.