renderer: implement new peak detection

This also allows us to finally separate peak detection from color
management. The current place in the code actually has almost no
drawbacks, since it's effectively free unless FBOs are disabled.

One annoying consequence is that this means we will now always perform
peak detection at the source resolution, even if the display is smaller.
In the relatively common case of 4K video on 1080p displays, this is a
performance regression. To fix it, we could try investigating whether to
do the analysis after up/downscaling, but then we have more special
cases to think about, so I think I'll live with the status quo for now.
Peak detection isn't the end of the world even at 4K.

Closes #40.