- Made it so that when a source is added or removed from a scene it
will add a reference to sourceSceneRefs (std::unordered_map). Each
source adds a reference to that every time they are added to a scene,
and releases a reference from it when they are removed from a scene.
When the value reaches 0, the source is no longer in any scenes, and
is then marked for removal and destroyed.
Before, I was using the source internal reference counter, which is a
really bad thing to do because I don't know what might actually be
referencing it. So using a separate discrete reference counter for
the number of scenes it's in is better in this case.
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