Allow BackgroundThread sharing between instances

This removes the need for every instance to have its own background
thread.

Cargo.lock

@@ -102,6 +102,12 @@ version = "1.0.66"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "216261ddc8289130e551ddcd5ce8a064710c0d064a4d2895c67151c92b5443f6"
 
+[[package]]
+name = "anymap"
+version = "0.12.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "33954243bd79057c2de7338850b85983a44588021f8a5fee574a8888c6de4344"
+
 [[package]]
 name = "approx"
 version = "0.3.2"
@@ -2601,6 +2607,7 @@ name = "nih_plug"
 version = "0.0.0"
 dependencies = [
  "anyhow",
+ "anymap",
  "approx 0.5.1",
  "assert_no_alloc",
  "atomic_float",