libobs: Should not clear tasks while resetting audio/video

[walker-WSH]

Description

While resetting audio or video, we should not clear the task of OBS_TASK_GRAPHICS&OBS_TASK_AUDIO.
It seems the tasks cannot be cleaned until obs_shutdown is called. When any module is calling obs_queue_task(wait=true), it will be blocked forever if the task is cleaned during resetting audio/video.

Note: My only concern is whether some tasks really do need to be cleaned when resetting audio/video

Motivation and Context

Clear task of OBS_TASK_GRAPHICS&OBS_TASK_AUDIO only in obs_shutdown

How Has This Been Tested?

1. add window/display source with WGC method
2. modify any video parameters, such as output resolution，format, colorspace
3. exit obs and ensure it can exit normally

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.