Unpair encoders on destruction + logs

[avoitenko-logitech]

Description

Attempt to fix crash in GPU encode thread. It seems like there are several execution paths that lead to this crash. This change makes attempt to fix one of them.

Motivation and Context

I noticed that during GPU encode data from paired (audio) encoder is used to check timings.
There is a function pair_encoders which sets paired_encoder for both sides (audio and video). But during call to obs_encoder_shutdown value is reset only from the one side => we have classic dangling pointer problem.

Also I added more logs for encoders lifecycle functions to help troubleshoot this nasty crash further.

How Has This Been Tested?

Manually in UI.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the streamlabs branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.