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.
