[SofaGui] Fix VideoRecorder

[hugtalbot]

Here is a fix for the VideoRecorder broken since #1121 due to change in API and in CMakeLists.

• Add findPackagae for ffmpeg since add_definitions does not propagate SOFA_HAVE_FFMPEG_EXEC above the caller project
• Change function initRecorder including both code extension and code name

Here you are @fspadoni ;)

---

This PR:

• builds with SUCCESS for all platforms on the CI.
• does not generate new warnings.
• does not generate new unit test failures.
• does not generate new scene test failures.
• does not break API compatibility.
• is more than 1 week old (or has fast-merge label).

Reviewers will merge only if all these checks are true.