Apply upstream patches to build FFmpeg's documentation with sys-apps/texinfo-6.8. 1. Fix missing CSS with texinfo 6.8 and above: https://github.com/FFmpeg/FFmpeg/commit/c980dd7a976635426f129417836251740e19b54b 2. Fix missing TOC with texinfo 6.8 and above: https://github.com/FFmpeg/FFmpeg/commit/bfbd5954e50e407693932b3900ca77c3daee26d7
See also https://bugs.gentoo.org/823197.