This accomplishes several things:

1. vec4 colors are now explicitly always normalized such that 1.0 is the
   reference white, and out-out-range values are used to communicate
   high dynamic range images. This makes a lot of processing steps
   saner, and doesn't matter much in the end. It also makes better use
   of the dynamic range of a float FBO, which is better for
   representational efficiency than linear FBOs either way.

2. HDR metadata now includes the frame average light level, not just the
   signal peak. This is important because it allows us to linearly
   darken over-bright scenes, which are sadly common due to shit
   mastering.

3. The tone mapping code takes into account the desired target peak and
   frame average light level. Not strictly necessary, but could be
   useful to play around with; especially for PQ output.

4. The peak detection SSBO code is fundamentally fixed. This had a large
   number of issues, too many to even bother listing. Now it should
   actually work properly.