Don't use VPIO forcelist by default (#244)

This makes it so clients have to opt into the VPIO forcelist.
It is currently mainly needed by gecko, since sites may create multiple
streams to a builtin device with different prefs and processing params,
and HAL audio units to builtin device are not working properly when
there is a VPIO audio unit to the same builtin device on (anywhere) on
the system.

With the cfg flag vpio-forcelist a client can avoid VPIO altogether, by
not compiling with the flag, and by never creating input streams with
the VOICE pref.

Should a client need to mix VOICE and non-VOICE input streams to a
single device, it should enable vpio-forcelist.

Fixes #242.

Cargo.toml

@@ -26,6 +26,7 @@ itertools = "0.11"
 
 [features]
 audio-dump = []
+vpio-forcelist = []
 gecko-in-tree = ["cubeb-backend/gecko-in-tree"]
 
 # Workaround for https://github.com/rust-lang/cargo/issues/6745 to allow this

src/backend/mod.rs

@@ -3266,6 +3266,7 @@ impl<'ctx> CoreStreamData<'ctx> {
     }
 
     #[allow(non_upper_case_globals)]
+    #[cfg(feature = "vpio-forcelist")]
     fn should_force_vpio_for_input_device(id: AudioDeviceID) -> bool {
         assert!(id != kAudioObjectUnknown);
         debug_assert_running_serially();
@@ -3283,6 +3284,11 @@ impl<'ctx> CoreStreamData<'ctx> {
         }
     }
 
+    #[cfg(not(feature = "vpio-forcelist"))]
+    fn should_force_vpio_for_input_device(_id: AudioDeviceID) -> bool {
+        false
+    }
+
     fn should_block_vpio_for_device_pair(
         &self,
         in_device: &device_info,