FFmpeg-derived portions used by Web-Demuxer

Web-Demuxer 4.0.0 states that its lib/ directory contains FFmpeg-derived
code licensed under the GNU Lesser General Public License.

GNU Lesser General Public License, version 2.1:
https://www.gnu.org/licenses/old-licenses/lgpl-2.1.txt

FFmpeg source code:
https://github.com/FFmpeg/FFmpeg

Web-Demuxer 4.0.0 source code and build scripts:
https://github.com/bilibili/web-demuxer/tree/4.0.0