openh264enc: Change default slice mode to auto
openh264enc: Change default slice mode to auto

This calculates the number of slices to use based on the number of
threads.

sancane/openwebrtc-gst-plugins,deepikateriar/openwebrtc-plugins-0.8,deepikateriar/openwebrtc-plugins-0.80,sancane/openwebrtc-gst-plugins,ford-prefect/openwebrtc-gst-plugins,ford-prefect/openwebrtc-gst-plugins,ossy-szeged/openwebrtc-gst-plugins,EricssonResearch/openwebrtc-gst-plugins,deepikateriar/openwebrtc-plugins-0.8,ford-prefect/openwebrtc-gst-plugins,ossy-szeged/openwebrtc-gst-plugins,deepikateriar/openwebrtc-plugins-0.8,deepikateriar/openwebrtc-plugins-0.8,ossy-szeged/openwebrtc-gst-plugins,EricssonResearch/openwebrtc-gst-plugins,sancane/openwebrtc-gst-plugins,EricssonResearch/openwebrtc-gst-plugins,deepikateriar/openwebrtc-plugins-0.80,deepikateriar/openwebrtc-plugins-0.80,ford-prefect/openwebrtc-gst-plugins,ossy-szeged/openwebrtc-gst-plugins,deepikateriar/openwebrtc-plugins-0.80