Previously, Xcode's main thread checker would complain when selecting
the "OpenGL Renderer" in the DemoRunner on macOS.

Fixes a bug introduced in 7d1918b385c600e1910dd5b744b583a42a26d8a1,
where Ozone 9's editor failed to open correctly in the AudioPluginHost.

The issue was caused by DropShadower using the ComponentListener interface
to listen to its target Component's changes and creating shadow Components
only if the target was visible during the event callbacks. However it was
possible that during the events the target was not yet visible because one
of its parents was not visible. When the parent became visible it would not
trigger a callback for the observed child component.

The fix attaches a ComponentListener recursively to all parents starting
from the target and responds to each componentVisibilityChanged() event.

Clang versions 5 to 8 inclusive seem to struggle to compile JUCE when
JUCE_BEGIN_NO_SANITIZE is used. This is unexpected, as simpler programs
using the same macro definitions compile correctly on clang 5.

Native Instrument VST3s (I tested Kontakt and Supercharger) crash when
the bundleEntry function is called from a background thread on macOS.
This change disables scanning for VST3 plugins on a background thread
using the PluginListComponent, to allow loading these plugins in the
AudioPluginHost.

I can't find any "official" word on whether the bundleEntry and
bundleExit functions should be guaranteed to be made from the main
thread. However, the VST3PluginTestHost app seems to call these
functions exclusively from the main thread.