build Debian CI with a stub for the decklink SDK

The stub was created from our source code, just to provide header that will compile. Running the code will just do nothing and return errors everywhere.

Debian was picked to use it because the other CIs will produce nightly builds that people might use and even the releases, which we don't want with this dummy module.

The contrib has to be forced manually, we don't try to detect if a legit SDK is available in the path.

It is using a newer value of BLACKMAGIC_DECKLINK_API_VERSION, but it's possible to force an older value in the command line to simulate older SDKs.

It is possible to enhance this stub to simulate a working hardware, that could be useful to test display modules in general. With extra work it might even be possible to reverse engineer the original SDK with the proper hardware and system DLLs installed.