On Windows, when opening a plugin editor, destroying the plugin
instance, and then creating a new instance and opening its editor, the
plugin would crash because the VBlankDispatcher singleton could not be
recreated.

Previously opening a PopupMenu and then clicking somewhere outside
the application would cause the mouse button representation to be
stuck in a down state.

In 6f3fb5a29f windowBorder member of
LinuxComponentPeer was changed to mean the logical size of the border
that is independent of the current scale factor. This was done to fix a
bug and make it consistent with the bounds member, which is also
independent from the scale factor.

This change wasn't taken into account in XWindowSystem::setBounds()
causing a positioning bug.

An animated drag operation will now stop if the user interacts with
the content area again before the animation is finished. It is also
stopped if the user interacts with the scrollbars.

Previously, positioning such an item would hang while trying to find an
appropriate position for the item, because no position in the grid was
suitable, and implicit cells in the layout direction would be added
until a viable position was found.

We now ensure that there are enough cells in the cross direction to hold
each of the auto-placement items before trying to position those items.

On Windows, when opening a plugin editor, destroying the plugin
instance, and then creating a new instance and opening its editor, the
plugin would crash because the VBlankDispatcher singleton could not be
recreated.

This patch also makes Optional public to end users.

The X42 stepseq.lv2 has an input atom port without the "control"
designation, but that still supports time:Position events. In order to
support this plugin, JUCE hosts will now send position info to any input
atom port that is marked as supporting this event type.

The host sync feature in stepseq.lv2 also requires the bar count to be
included in the position information.

Observed on Ubuntu Linux. Occasionally, the loop checking the
condition_variable in the plugin scanner would spin indefinitely.

The cause appears to be that handleMessageFromWorker could be
called immediately after sendMessageToWorker, but before locking the
mutex. If this happens, gotResponse will be false during every call to
condvar.wait_for, and the loop will never exit.

The rewritten version of the scanner always resets gotResult immediately
after the condvar is woken successfully, so a call to
handleMessageFromWorker or handleConnectionLost will always cause a
subsequent call to condvar.wait_for to exit successfully.

The Superprocess class has also been refactored and extracted to avoid
a circular dependency between Superprocess and CustomPluginScanner.

Previously, activateBus would fail if the new BusesLayout wasn't
supported, as reported by isBusesLayoutSupported. However, according to
the VST3 docs, a host is allowed to enable and disable buses in any
combination, and the plugin should be able to handle this gracefully.

The ability to enable/disable individual buses without failure is
particularly important because there's no VST3 API to set a complete bus
layout in one go. That is, the only way to set all buses active or all
buses inactive is to set the appropriate state on each bus individually,
which in turn means that at some point, some buses will be active and
some will be inactive. Disallowing such 'intermediate' states may
prevent the host from putting the plugin into other (valid) states.

To ensure that the VST3 wrapper always accepts activateBus calls, it now
keeps track of the activation state of each bus as requested by the
host. When the host tries to change the activation state, the wrapper
will try to set the host's "ideal" bus layout on the AudioProcessor. If
this fails, the AudioProcessor will retain its previous bus layout.

The buffer remapping inside the process callback has been made more
robust, to handle cases where the host and the AudioProcessor disagree
about the activation state of each bus:

For input buses:
- If the host has activated the bus, but the AudioProcessor decided to
  keep the bus inactive, the host's input will be ignored.
- If the host deactivated the bus, but the AudioProcessor wanted to keep
  the bus active, the AudioProcessor will be provided with silence on
  that bus.

For output buses:
- If the host has activated the bus, but the AudioProcessor decided to
  keep the bus inactive, the wrapper will clear the host's output
  bus buffers.
- If the host deactivated the bus, but the AudioProcessor wanted to keep
  the bus active, the AudioProcessor's output on that bus will be
  ignored.

The AudioBuffer passed to the wrapped AudioProcessor will no longer
contain any pointers from the host's ProcessData. Instead, the host's
inputs will be copied (in JUCE channel order) to a temporary buffer,
and this temporary buffer will be passed to
AudioProcessor::processBlock. After processBlock, the buffer contents
will be copied to the host's output buffers.

This change is intended to avoid a potential issue when reordering
channels into JUCE order, which may necessitate copying a host input
channel to a different host output channel. In the case that the host is
using the same buffers for both inputs and outputs, copying an input to
an output channel may end up overwriting another input channel, breaking
the plugin's inputs.