[video-codec] Command-line options for encoder comparisons

[Thomas Daede <[e-mail]>]

I am updating the scripts that AreWeCompressedYet uses. I would like to
select what rate control options are used in the comparison. This is for
purposes of absolute comparison between different codecs, and to
indicate when a particular performance point is reached. This is not for
adding tools to an existing codec. There are a couple of different rate
control modes that we can use to compare:

Constant Quantizer:
This is not as simple as it might sound. Should referenced frames still
be allowed a boost in this mode? Should adaptive quantization be
allowed? Can quantization matrices be switched? I tried to come up with
a sane way to compare different codecs with a constant quantizer and was
not successful.

Constant Quality:
Here the codecs are set to use all features available within temporal
constraints. For stored video, this means full lookahead (or two pass,
if that's what the codec uses). For realtime, this means no lookahead,
but otherwise all quality-oriented rate control is allowed. Example rate
control parameters:

VP9 parameters:
vpxenc --codec=vp9 --cq-level=$x --end-usage=q

x264 parameters:
x264 --crf=$x

x265 parameters:
x265 --crf=$x

Constant Bitrate:
This is the hardest category to compare, as it means that all of the CBR
implementations need to be set to have the same buffer constraints. It
is an important use case, so I think we will need to figure out a way to
test it. I'd be okay with adding this one later, though.