macosx/framework: Make sure the nsobject drawable is never used after being freed.

macosx/framework: Make sure the nsobject drawable is never used after being freed.

This issue was pointed out by Sébastien Zwickert.

We ensure this by two things:
- Retaining it by the media player and ensuring we are effectively stopped when releasing it. (ie, no one will use the drawable from now on).
- Retaining it during the life span of the vout. After a stop, the drawable might still be in the progress of receiving the notification of the vout removal, so we need it not to be freed during this period of time.
An alternative would be to create a protocol between drawable->vout to unregister the drawable.