openh264: add slice support
Copied from https://gitlab.freedesktop.org//gstreamer/gst-plugins-bad/-/merge_requests/741
This MR adds the support of openH264 slicing mode.
$ gst-launch-1.0 -v videotestsrc num-buffers=10 ! openh264enc num-slices=8 ! openh264dec ! autovideosink