This fixes warnings that are emitted when building with the `-analyze`
flag enabled.

In cases where a VST3 plugin set a non-zero latency in its
prepareToPlay, and then set its latency to 0 later on, the host was not
notified about the latency change.

In older versions of the VST3SDK, hostContext is a raw pointer,
in newer versions it's a smart pointer. If we do manual
refcounting with the smart pointer, we may cause leaks.

Previously, IEditController parameter indices were being used to index
into the AudioProcessor parameter array, but these parameter indices are
not guaranteed to point to the same parameter (parameter groups may
cause reordering on JUCE's side). Now, we use the IEditController
indices universally.

Audition incorrectly increments the ref-count of the IPlugView* returned from IEditController::createView() and never fully releases it. This commit reverts e0306d25 for Audition to ensure that an editor is always returned, otherwise Audition displays a blank GUI.

MIDI FX AudioUnits can retrieve their current sample rate from an output
bus. However, it's reasonable for a JUCE MIDI FX plugin to require 0
output buses, in which case the correct sample rate cannot be retrieved.

This patch modifies the AU wrapper to ensure that MIDI FX plugins will
always have at least one output bus. This bus does not necessarily
correspond to a matching bus in the AudioProcessor, and it will not be
passed to AudioProcessor::processBlock. With the output bus in place,
the correct sample rate is reported in `prepareToPlay`.

Previously, all VST3 parameter changes were signalled to the host via
performEdit, regardless of the originating thread. This breaks the
contract of the IEditController interface, which requires that all calls
are made from the UI thread.

This change modifies the VST3 wrapper so that it will only call
performEdit when a parameter is set on the message thread. If a
parameter is set on the audio thread instead, then the parameter change
will be signalled to the host using
`ProcessData::outputParameterChanges`.

If your VST3 plugin uses a background thread to update parameters for
some reason, you'll encounter data races. Please don't do that.

When loading a JUCE plugin in Live, Live was producing diagnostic
messages in its Log.txt warning that restartComponent was being called
from a non-UI thread. This change should silence this warning, and
improve compliance with the VST3 spec.