fix: SG-43388: RV leaks memory because of opened decoders

[deltag0]

fix: RV leaks memory because of opened decoders

Summarize your change.

• Re-enable old context map that manages the number of open threads to decode videos

Describe the reason for the change.

• Without the context map, the number of open decoder threads keeps increasing as we load more videos.
• RAM usage would increase a lot even by loading video clips without playing the video at all, and just staying on the first frame.

Describe what you have tested and on which operating system.

Tested on CY 2025 on Rocky Linux 9.8 and Mac

Opened over 1000 clips with 1GB of caching, peaking at 9GB, but staying bounded.
RAM no longer increases when loading clips without playback being on.

Add a list of changes, and note any that might need special attention during the review.

• After freeing the decoder with avcodec_free_context, nullify the tracked pointer to not point to freed memory
  No regressions by adding the context map

If possible, provide screenshots.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: deltag0 / name: deltag0 (686e739, 9c09807)