Freeze x264 version.

[Zalewa]

Currently all libraries are fixed on a specific version except for the x264 library for which the "last" snapshot is always downloaded. This means that the ffmpeg build is non-deterministic and the resulting ffmpeg binaries depend on the day on which they were compiled, even though the version of ffmpeg itself is fixed too. Moreover, the whole build can suddenly break if the "last" snapshot of x264 changes something that breaks the current build scheme.

I propose to pick a specific x264 version and stick to it for the time being and only update it manually once in a while under controlled circumstances where it will be possible to verify if the new version breaks something. Given that all snapshots are good, picking any snapshot (even the newest one) will be fine.

[zimbatm]

Yes, this is a great idea.

Bonus points for adding a little script that resolves the ref sha1 from the branch for you and thus makes it easy to bump the version.