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Use shared_ptr for plugin instances throughout; also descriptors are copyableaudio-source-refactor 2020-03-20, by Chris Cannam
Merge from branch background-mode2020-01-24, by Chris Cannam
Add optional unset entry to colour combobackground-mode 2020-01-24, by Chris Cannam
Make nearby-colour matching a bit more sophisticated, with slightly better algorithm (we think?) and ability to insist on dark/light backgroundbackground-mode 2020-01-24, by Chris Cannam

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