Add two-pass encoding

[RealOfficialTurf]

I've changed the script to utilize two-pass encoding and changed the encoder from libvpx-vp9 to libx264. The two-pass encoding should make it so that the encoder can optimize parts of videos with varying bitrate while staying within the average bitrate target.

EDIT: Turns out you can also do that in libvpx-vp9. I've reverted the encoder back to libvpx-vp9.