This will add a fallback mechanism that uses a
normal layer-backed NSView with a dark but still
slightly transparent background on macOS versions
that do not support NSVisualEffectView.

Additionally it will allow compilation with older
SDK's that have no NSVisualEffectView defined.
If compiled with such an SDK, support for
NSVisualEffectView is disabled completely, so
even when running the compiled VLC version
on 10.10 or higher it will use the old fallback
view, as the SDK it was compiled with had not the
necessary definitions.