C++20 introduced designated initializers similar to the ones found
in C99, however, in C++ designated initializers cannot be mixed
with non-designated initializers. GCC rejects mixed initializers
with an error.
Find a free id from the available ids instead of just taking the current
number of devices, which would give assign the same id to multiple
cameras when: added 0, added 1, removed 0, added 1.
Previously, if "add_listener" was called on the monitor device, then it
did not necessarily emit events about all devices because it called
`enum_devices()` which does not emit events about already existing
devices. So the very first listener would get all existing devices,
but subsequent ones would not get events about the existence of devices
that have already been seen by the monitor. Fix that by simply emitting
events about all existing devices if the current listener is not the first.
libcamera's CameraManager runs the event handlers on its own thread,
thus synchronization is needed to ensure correct functionality.
Implement that by collecting hotplug events into a mutex protected queue
and signalling the main loop where they can be processed and the
"object_info" event can be safely emitted.
Using a shared_ptr removes the need for manually calling
`libcamera_manager_release()` to drop the reference as it is done
automatically whenever the shared_ptr is destroyed or reset.
Previously, the libcamera manager `impl` object was neither properly
constructed neither properly destructed. As a consequence, for example,
the shared pointers in the `devices` array weren't properly destructed,
although this has been somewhat mitigated by a previous change
that modifed `clear_devices()`.
Previously, in `remove_device()`, the last device would be copied into
the slot of the to-be-remove device. The problem with this is that it
left the shared_ptr untouched in the previously last slot, and hence
creating an extra reference. Fix this by moving instead of copying.
A similar problem is present in `clear_devices()` which also
did not properly dispose of the shared_ptrs. Fix that by
calling `reset()` on each device's camera pointer.
