GSCapture: add volume slider and scale recorded audio

[SternXD]

Description of Changes

Fixes #12700

• Added an audio capture volume slider
• Made a new AudioCaptureVolume setting stored in GS config, defaulting to 80%.
• Applied the volume multiplier when feeding samples to the FFmpeg encoder

Originally I was going to add libavfilter but I couldn't justify adding that library for one feature so that's why I did this the way I did.

Rationale behind Changes

Giving users a volume control makes built-in recording behave more like standard capture tools where mic/system levels can be balanced before hitting "Record."

Suggested Testing Steps

• Start a capture with default settings, then inspect the resulting video to confirm the recorded audio is quieter than before.
• Change the slider to e.g. 50% and 100%, capture again, and confirm the output volume scales accordingly for the video capture

Did you use AI to help find, test, or implement this issue or feature?

No.