/usr/lib/x86_64-linux-gnu/libwebpdemux.so is in libwebp-dev 0.6.1-2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libwebpdemux.so.2.0.3
/usr/lib/x86_64-linux-gnu/libwebpdemux.so.2.0.3