Prevent h26x from letting through invalid frames after frame loss

[jrsnen]

I think at least one designed feature was to drop inter frames until the next intra in case of lost frames, but at the moment, this does not seem to prevent invalid frames from getting through (inter frames refer to previous intra frame and cannot be displayed if a previous frame or an intra frame is missing).

[jrsnen]

It would also be nice if this feature was hidden behind a flag.

[jrsnen]

I'm actually unsure at this point whether this works currently already. Further testing should be on what happens when the network drops packets. There is supposed to be this feature that drops all undecodable frames. I would suggest fixing #105 before testing (working on it currently).

[jrsnen]

There is a simple version of this in the master, but it is not yet a complete solution (removed the old system which did not work). It can be disabled with a flag.

[jrsnen]

I disabled the current system by default because it won't work with non-monotonic timestamps, and it is not yet completed.