I tested version 5.0 with libav-11.3 and it works (build an run a Game). Dependence (!!>=media-video/libav-10) in the ebuild is no longer necessary.
This has been fixed and will be in the tree shortly. I'll close this tomorrow unless I get some notice in here that it doesn't work post-modification. :) Thank you for reporting!
> ffmpeg? ( virtual/ffmpeg > !!media-video/libav ) Is that the fix? That's even worse than before.
Let's try this a second time.
Now the Dependencies OK