The two fudge factors in the reverb path cancelled, which is why the
overall level felt roughly right.
- The send is accumulator * 0x20 >> 16, i.e. sample >> 2. We used >> 1,
driving the reverb 6dB hot.
- The return is (evol * out) >> 11. We used >> 12, i.e. 6dB quiet.
Net level is therefore unchanged, but the reverb now runs at the level
the presets were designed around. That matters because the filter clamps
internally, so a 6dB hot input changes how the feedback path saturates -
worst on the presets with heavy feedback.
Also adds a slider in the imgui.