* Change handling of empty gbuffers.
Now, instead of sending an empty image, a black one with the size of the viewport is sent if the target gbuffer is unavailable.
* Add more GBufferID checks + improve empty gbuffer handling.
* Fix manual_control_gbuffer.py error on repeated listen_to_gbuffer.
* Expose is_listening and is_listening_gbuffer to the PythonAPI.
* Fix data race + autoremove unused gbuffers in manual_control_gbuffer.
* Update PythonAPI docs.
* Remove magic number.
* Switch from error to warning on invalid sensor type when requesting gbuffers.
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