Dynamic latency change: where we stand

# Would dynamic latency be possible, and what would it take?

_TL;DR it's not always doable, nor desirable!_

What are the pluses and minuses?

An ultimate goal would be to allows a signal sub-graph to have latency reported **at any time**, not necessarily at **initialization** or even **before-hand** (like it is today).

However every format is a bit different with regards to latency.

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## VST2
When receiving a "Resume" opcode, VST2 allows is to compute latency there and update it.
Other developers report that all hosts follow this correctly.
A parameter that changes latency has to be marked with `nonAutomatable()` in `buildParams()`. (EDIT: not all plugin do that)
However, Voxengo says that users don't like a parameter that need transport restart.
**While transport is on, in VST2 no latency change will be applied and PDC will be wrong until the user restart it.**

That's a key reason why many plug-ins currently _pessimize_ the latency and then compensate internally to the worst case (Auburn Sounds, Voxengo... and probably others).

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## VST3
Is it unknown to us how much VST3 hosts like this dynamic latency thing, however the standard allows to do it at play time `setupProcessing`.
I guess we _could_ try make some parameter call `_handler.restartComponent(kLatencyChanged);` if they change latency, but as noted later, users would probably not like it.

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## AU
Audio Unit allows latency change pull-mode from the host with the property `kAudioUnitProperty_Latency`, listeners, etc. Reajusting is relatively inaudible in Logic, I don't rememember for other hosts but at least it compensated. Unknown how much dynamic that can be.

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## AAX
Very clear documentation on this one (cannot reproduce here).
`virtual AAX_Result AAX_IACFController::SetSignalLatency`
In short:
- you can request a latency change, seemingly at any time
- plugin then receive a notification when this happens (unknown if this happens while transport is playing, like Logic)
- again, recommendation that a parameter that changes latency is non-automatable

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## LV2

No investigation yet

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Voxengo described how they do:
- dynamic latency change for AAX and AU
- worst-case latency for VST2 and VST3

Start by comparing the latency-change behavior described for VST2, VST3, AU, AAX, and LV2, then inspect the plugin-format integration points in Dplug. Determine whether dynamic latency can be supported consistently and document the trade-offs, affected formats, and scope of any implementation.

Written by the indexing model from the issue text.