Monitoring plugins not properly supporting shared inputs

[firthm01]

Original Title: Monitoring issue when a DS layout is changed

E.g,

• Import ESC_short_ADM.wav with EAR
• Without opening the Scene, hit play - sounds correct (bed + eng comm in centre as that the default programme)
• Change the layout of the bed in the DS plugin to 22.2 - we expect some oddity as we're now telling the scene that channels 1-24 are all for the bed, where as actually it's receiving eng com on 11 and ger comm on 12 - so they're going to get treated as legs of the 22.2 bed.
• Open the Scene. Notice how the panning of the commentary changes - something has triggered an update to the monitoring plugins.

EDIT: This reasoning is not entirely correct - see later comments

This is because routing changes are currently only detected if the routing parameter changes (i.e, starting channel). A change of layout is not detected as a routing change, even though it should be. See this bit of code; https://github.com/ebu/ear-production-suite/blob/9a5c788421e242a47364567258b1b3b304f12cfe/ear-production-suite-plugins/lib/src/scene_store.cpp#L90-L94 ~~This triggers flagOverlaps which adds all of the items to the itemsChangedSinceLastSend vector, making sure the monitoring plugins update their state for all items (we kept it simple here rather than trying to figure out exactly which items are affected). Opening the Scene plugin triggers flagOverlaps via another code route which is why we hear the correction only when the Scene plugin is opened.~~

[firthm01]

Regardless, the affected item should have it's change flag set by the input plugin, and so this should acted upon by the monitoring plugins - to check. EDIT: The change flag is set and is received by the monitoring plugins as expected.

[firthm01]

The problem is that the SceneGainsCalculator expects only one use of an input channel. When metadata for an AudioObject is received, the matrix (direct_ vector) for those input channels are updated. If another AudioObject also uses those input channels and sends metadata, the matrix is then updated to represent that AudioObject, overwriting the previous values.

This section of code runs once for every piece of AudioObject metadata received; https://github.com/ebu/ear-production-suite/blob/9a5c788421e242a47364567258b1b3b304f12cfe/ear-production-suite-plugins/lib/src/scene_gains_calculator.cpp#L123-L129 You can see how it would overwrite whatever was previously stored in direct_[inputChannel] if inputChannel is reused between AudioObjects. Gains should be a summation of calculations for all AudioObjects using that input channel - perhaps we need to do per-object calculations and then sum whenever the direct or diffuse matrices are requested by the audio processing backend.