videodecoder: Add API for subclasses to request a new sync point

videodecoder: Add API for subclasses to request a new sync point

This allows subclasses that notice missing reference frames to request a
new sync point to allow seamless decoding again. While doing so the
subclass can also signal whether it wants a) all following input frames
until the sync point to be discarded or b) all output frames until the
sync point to be marked as corrupt.

Sending of force-keyunit events for this can be throttled by the
application via the "min-force-keyunit-interval" property.

This replaces custom behaviour for the same in various decoders, for
example openh264dec.

Based on patches by Haakon Sporsheim <haakon@pexip.com> and
Stian Selnes <stian@pexip.com>.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/730>