Add limiter module

[falkTX]

Something missing in Cardinal. excerpt from IRC talk, for future reference:

00:54 < jtb> Another major thing Cardinal needs is a good limiter, the bogaudio one lets through fairly big peaks even with 0 attack.
00:54 < jtb> I remember you asked what was missing the other day so I've been thinking about that
01:08 < dreamer> find us one
01:08 < falktx> zamaudio has one I think
01:08 < falktx> or we can just reuse x42 dpl plugin in rack
01:09 < falktx> https://x42-plugins.com/x42/x42-limiter
01:10 < x42> would not be the worst idea
01:11 < dreamer> we just need a vertical UI ;)
01:11 < falktx> x42: what is the minimum block size the plugin can run at?
01:12 < x42> 1, but it has latency
01:12 < x42> since it's a look-ahead limiter
01:12 < falktx> that is expected
01:13 < falktx> jtb: note both Rack and Cardinal have a 0dB (-/+1.0f) brickwall limiter on the outputs
01:13 < x42> no idea how well it performs, but the inner loop isn't vectorizable to begin with (there are branches), so it's probably fine.
01:15 < falktx> x42: any parameters are suitable for audio-rate modulation?
01:15 < x42> falktx: there is a "cleaner" implementation at https://github.com/x42/sound-gambit/blob/master/peaklim.cc#L197-L350
01:17 < x42> yes, gain is interpolated. threshold is intentionally not. when you change it you want it to directly hard-limit
01:17 < x42> and release is a time-constant which doesn't need interpolation, since it only indirectly affects the signal
01:18 < falktx> so gain and release can have cv modulation
01:18 < falktx> should put this info in a ticket..
01:18 < x42> in theory so can the threshold. might be interesting
01:19 < x42> that probably results in constrained amplitude modulation. 
01:20 < x42> it's not what you'd usually use a limiter for, but why not?!