Active camera is a property of Scene, so need to take scene changes into
account for such drivers to work reliably.
The fix covers all the common cases of such configurations, but some of
them might not be yet fully supported. Mainly cases when the target ID
is not covered by the copy-on-write mechanism.
There is a fuller explanation available in the code for the ease of reading
by the future generations.
Differential Revision: https://developer.blender.org/D15146
83fd3767d3