contribs: really fix ffmpeg compilation on Darwin (by reverting most of [25028] and adapting its 2nd hunk to fit the upstream code)

fix zsh completion generator, add a script to help compilation, generation, and installation of completion

* modules/codec/x264.c: add satd exhaustive motion search (tesa) to the list of motion estimation algorithm options

* modules/codec/x264.c: set default for direct prediction size (direct-8x8) to -1 (smallest possible according to level); this fixes the direct 8x8 inference warning on level requirement in certain x264 setting combinations