MIDI only plugins are no longer provided valid audio buffers
Graph nodes are only executed when they have active connections

Previously it was possible to have a dangling resource map. In that case
opening an AU from a file that did not have a resource map would lead to
using the previously unclosed map and attempting to load the previous
plugin again.

fc378aaf9a introduced a regression where
plugins with no audio channels (such as MIDI FX plugins) would receive
an audio buffer with a length-in-samples of '0', rather than the actual
block length.

These were mistakenly removed in afe5199848

Fixes an issue introduced in 72fa2d98e1
where zeromem might write past the end of the channel buffer.

If the block size changes from block to block, then it's possible for
inputBuffer to be smaller than buffer, but for buffer to be smaller than
the initially-allocated size of inputBuffer.

We haven't updated the default post-build copy paths yet, as the new
local directory is not yet well-supported by DAWs.

When plugins change their 'parameter info' this often means that their
previous parameter text is no longer valid, so we should fetch new text
to represent the parameter value.

This change was primarily tested with Kontakt.

This change was tested with FabFilter Pro Q 3, which supports new AU
channel layouts for Atmos.

With this patch applied, the DemoRunner should build under MinGW, and be
(nearly) feature-complete compared to the MSVC build.

Specifically, when building with MinGW:
- Adds support for accessibility
- Fixes build issues in the juce_video module
- Fixes a link issue in the VST3 wrapper when VST3_CAN_REPLACE_VST2 is
  defined
- Adds support for the new-style native FileChooser
- Tidies up some other low-severity warnings

Known issues:
- Direct2D rendering is still not supported when building with MinGW due
  to ABI compatibilities.