This fixes an issue with recent JUCE when using a stereo
plugin on a mono track.
`juce::validateLayouts` requires that numChnanels matches
the configured channel-mapping:
69795dc8e5/modules/juce_audio_processors/format_types/juce_VST3Common.h (L630)
JUCE could be more liberal and accept
`(int) mapIterator->size() > it->numChannels`