obs-vst: Fix crash caused by mult-thread accessing to VSTPlugin::effect

[walker-WSH]

Description

fix thread-safe issue for effect

VSTPlugin::unloadEffect() is called from UI thread. VSTPlugin::process is called from audio source

Motivation and Context

fix crash

How Has This Been Tested?

select a vst for audio source; keep switching vst

Types of changes

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

Checklist:

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

[jp9000]

Seems to have a conflict since the other PR was merged for closeEditor.

[walker-WSH]

Seems to have a conflict since the other PR was merged for closeEditor.

Conflict is fixed. I have to fix clang error tomorrow because no environment now.

[walker-WSH]

@jp9000 It is ready to be merged now