Some AUv3 presets crash when querying the set of presets in Loopy Pro.
The issue seems to be because `addPresets` may end up being called
concurrently with the host's queries.

Some AUv3 presets crash when querying the set of presets in Loopy Pro.
The issue seems to be because `addPresets` may end up being called
concurrently with the host's queries.

Previously, preparing an AudioProcessorGraph containing hosted LV2
plugins would recreate the plugins and then set their state. For plugins
without threadsafe restore, setting the state would take the
callbackLock to avoid races with processBlock. This meant that

- During prepare, the graph would take the processorLock, then the
  processor would take its own callbackLock.
- During playback, the graph would take the processor's callbackLock,
  and then would take the node's processorLock.

This is probably benign (prepare shouldn't be called concurrently with
processBlock at all), but to be on the safe side we now avoid taking the
callbackLock when setting new plugin state during prepareToPlay.

This is an implementation detail for the build system of the plugin.
Users of the plugin should not know or care that this extension is
available.

Also updates the drop shadower so that the DPI-awareness of the shadows
matches the DPI-awareness of the shadowed component.