Was tired of seeing magic number 1000000000LL everywhere, so I made it a type

The reason for this number is that most libweb time functions do time
in nanoseconds. If a functions does TimeCode, TimeCode = Time(ns) /
timeCodeScale
MPEG-2 chroma siting

Seems like WebM uses MPEG-2 chroma siting, which means the site is
between the current pixel and the one below. So we'll average the two
together instead of just using the current pixel.

Note that I'm not really handling interlacing, which uses different
citing from this, I think.
Fix chroma sampling bugs with odd dimensions