Fix MacOS video renderer disposing

[evdokimovs]

Synopsis

At this moment we have bug which is easily reproducible in our getUserMedia example.

Steps for reproduce:

1. Start GetUserMedia demo
2. Change video camera to any camera (it can be the same camera)
3. Change video camera second time
   => segfault

Solution

Fix memory management issues in MacOS's implementation of VideoRenderer.m.

Checklist

• Created PR:
  • In draft mode
  • Name contains issue reference
  • Has type and k:: labels applied
  • Has assignee
• Before review:
  • Documentation is updated (if required)
  • Tests are updated (if required)
  • Changes conform code style
  • CHANGELOG entry is added (if required)
  • FCM (final commit message) is posted or updated
  • Draft mode is removed
• Review is completed and changes are approved
  • FCM (final commit message) is approved
• Before merge:
  • Milestone is set
  • PR's name and description are correct and up-to-date
  • All temporary labels are removed

[evdokimovs]

FCM

Fix double free when macOS video renderer is reused (#139)

Additionally:
- fix TextureVideoRenderer not being unregistered from FlutterTextureRegistry