OSS-Fuzz: Add new fuzzer targets scene loading

[arthurscchan]

This PR adds a new OSS-Fuzz fuzzers for scene loading logic.

[aureliendavid]

Hello,

Thank you for the contribution.

We continuously try to improve fuzzing coverage and overall memory safety in gpac.

However, with limited time and resources we have to be strategic with our priorities.

The recently added fuzz_scene harness that targets really old code has thrown many oss-fuzz issues and has overwhelmed my schedule for days.

The one in this PR targets similarly old gpac modules that will likely generate a bunch of memory issues.

Although I agree that these would need to be fixed, the gains for actual end users are very limited, but the time commitment is significant and I can't really afford it right now.

So I will merge this at some point, but not right now. It'll have to wait for a calmer time.

(this is not a reproach on your PR, it is still very welcomed to receive contribs and increase coverage, I just wanted to explain my thinking instead of just leaving this in limbo, hopefully that's understandable)

Thanks again.

[arthurscchan]

No problem. Thanks for your prompt reply. Do let me know if you need any clarification of the fuzzer we wrote.