libcamera: signal: Fix Object handling in multiple inheritance cases

The SlotBase implementation stores the receiver object pointer as a void
pointer internally. The pointer is then cast back to an Object pointer
when the receiver object class derives from Object. When the receiver is
an object that inherits from both the Object class and other classes,
the Object data members may not be stored at the beginning of the object
memory. The cast back to an Object pointer is thus incorrect.

Fix this by casting the receiver object pointer to an Object pointer
where the type of the receiver object is known, and pass it along with
the receiver void pointer to the SlotBase class. The SlotBase class
stores both pointers internally, and doesn't need the isObject_ field
anymore as the same information is obtained from checking if the Object
pointer is null.

To avoid confusing the two pointers, use the same naming scheme through
the whole implementation: "obj" points to a receiver object as an
unknown type, and "object" to the receiver object cast to an Object. The
latter is null when the receiver object doesn't inherit from the Object
class.

To further clarify the code, remove direct access to the SlotBase "obj"
and "object" fields as much as possible. They are replaced by two new
methods :

- SlotBase::disconnect() to disconnect a signal from the slot's receiver
  object
- SlotBase::match() to test if an object pointer matches the slot

The match() method is a template method with a specialisation for the
Object type, to compare either the obj or the object pointer depending
on the type of the parameter. This is required as the Object destructor
calls the SignalBase::disconnect() method for signal connected to the
object, and passes a pointer to Object to that method, while the actual
object may have a different address due to the issue explained above.
The pointer must thus be compared with the stored Object pointer in that
case, not to the pointer to the receiver object.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>

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