plugins/compressor: Fix ducking trigger when sidechain source is muted

[ShivaPlays]

Description

This PR addresses the issue where the compressor continues to duck audio based on its last known envelope state when the sidechain source is muted or becomes inactive. The implementation introduces a graceful release decay using the configured release time when the sidechain source is unavailable or muted, preventing sudden volume jumps and ensuring consistent gain behavior. It also preserves standard self-compression functionality when no sidechain is configured.

Motivation and Context

This change fixes #13170.
Currently, when a sidechain source is muted in the OBS audio mixer, the compressor retains its previous gain reduction level instead of releasing. This results in audio remaining "ducked" even when the sidechain is muted or inactive. This fix ensures that the compressor behaves predictably by treating an inactive/muted sidechain as a signal for a natural release curve.

How Has This Been Tested?

I have verified this fix on a local build of OBS Studio.

• Test Case 1 (Sidechain): Created an audio output source with the Compressor filter using a sidechain. Observed that muting the sidechain source now triggers a smooth release decay instead of holding the gain reduction.
• Test Case 2 (Standard): Created an audio input source (microphone) with a Compressor filter without a sidechain. Verified that standard self-compression remains fully functional and unaffected by these changes.
• Environment:
  • OS: Kubuntu 26.04 LTS
  • KDE Plasma: 6.6.4 | Kernel: 7.0.0-15-generic
  • Hardware: AMD Ryzen 7 4800H, NVIDIA GTX 1660 Ti

Types of changes

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

Checklist:

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