We cannot rely only on the DISCONT flag when deciding whether we need to
request a new key frame or not because it might be that the packet that just
came in with the DISCONT flag is actually the start of a keyframe.
This patch improves the logic to be the same as rtph265depay, by only requesting
a key frame if the packet with the DISCONT flag is not the first one of a FU; or
if its the first one, only request it when we could drop packets due to a
missing key frame.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8343>
