Feature Request: Volume slider

[franzmondlichtmann]

Sets the volume with a slider.
Keybinding for the slider with + / -
The use case is that I want to have other videos on the background (multiple audio streams at once).
And I don't want the tracker music to be too loud.

[kajott]

Implemented in da296dd.
There has already been an option like this, in the form of the gain parameter in the INI file, but since that can't be adjusted at runtime, an additional volume control stage has been added.