obs-ffmpeg: Fix deadlock on shutting down muxer

[exeldro]

Description

The things that happen to get this deadlock:

• the io thread reads shutdown_requested as false
• the main thread sets shutdown_requested to true
• the main thread gets the data_mutex
• the main thread signals new_data_available_event
• the main thread releases the data_mutex
• the main thread waits for the io thread to exit
• the io thread gets the data_mutex
• the io thread resets new_data_available_event because chunk_used < 65536 && !shutting_down
• the io thread releases the data_mutex
• the io thread waits for new_data_available_event

with the changes the new_data_available_event cannot be send from the main thread between the reading of shutdown_requested the getting of the data_mutex in the io thread

Motivation and Context

Got a memory dump Fenrir from a muxer that was stuck.
the main thread is in ffmpeg_mux_free waiting for the io thread to close.
the io thread is waiting for new_data_available_event

How Has This Been Tested?

Muxer still works as expected

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 master 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.