Declares every asset this component needs, and keeps each returned
handle in a field - the third declare-time pass, chained through mixins
with @mustCallSuper exactly like describeType and describeStruct.
The scene the entity this component instance describes was registered
with - reachable from any mixin on Component/on MultiComponent, not
just from the top-level EntityStruct subclass, since EntityStruct
adds nothing to Component's own interface that a mixin's on bound
could see. EntityStruct is the only implementation; every mixin just
inherits visibility into it through this interface. Throws if the
running scene isn't a T - see EntityStruct's implementation.
