Code cleanup in the rendering engine:
- Partial::produceOutput() now mixes samples into the buffers in-place
- temp partial rendering buffers nuked
- other temp buffers became local and are allocated on the stack thus no longer consume memory
- DACInputMode stuff reorganised in more convenient manner
- other code cleanup
- Removed seemingly unused method MidiEventQueue::popMidiEvent()
- MidiEventQueue::dropMidiEvent() added instead