Add a separate audio thread mutual recursion stack

This should fix #118 without breaking our _other_ workaround from
yabridge 3.4.0 to fix the issue where a plugin would freeze if it would
try to resize itself while at the same time it sent parameter changes
from the audio thread. (and both of these issues of course are caused by
the same JUCE bug)

CHANGELOG.md

@@ -6,6 +6,13 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic
 Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Fixed
+
+- Fixed a regression from yabridge 3.4.0 where JUCE-based VST3 plugins might
+  cause **Ardour** or **Mixbus** to freeze.
+
 ## [3.4.0] - 2021-07-15
 
 ### Added