/usr/lib/x86_64-linux-gnu/libwebpdemux.so is in libwebp-dev 0.5.2-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libwebpdemux.so.2.0.1
/usr/lib/x86_64-linux-gnu/libwebpdemux.so.2.0.1