videoencoder: Handle all matching force-keyunit events at once

videoencoder: Handle all matching force-keyunit events at once

Previously we only handled one event at a time, which could lead to the
following two suboptimal situations:
- frame 0 at 20ms, frame 1 at 40ms and two force-keyunit events at 10ms
  and 15ms. We would create a new keyframe for both of the frames.
- 100 force-keyunit events with running-time NONE would cause all
  following 100 frames to be made into a keyframe.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/684>