Bug#1004616: performous: FTBFS with ffmpeg 5.0

[Dan Bungert]

Dear Maintainer,

I did a test build using the version of performous in experimental on Sid and
Ubuntu Kinetic.  I found that it continues to have a const-related build
failure.

Upstream commit
https://github.com/performous/performous/pull/752/commits/80d5b08d34a97db16fe12f82e9060570087ef0f0
is a step in the right direction, but in this older codebase needed more.

Please see attached for a patch that fixes build of the version in experimental
with Sid and Ubuntu Kinetic.

-Dan