Reintroduce volume Knob using PortMixer

[mixxxbot]

Reported by: daschuer
Date: 2015-04-05T00:45:01Z
Status: New
Importance: Wishlist
Launchpad Issue: lp1440443

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Like Audacity, Mixxx should allow to fade to change the volume of the master output without loosing bitwidth and introducing noise. This allows to get rid of the external fader. And avoids complaining about bad sound at low volume.

The PortMixer API can be found here:
https://github.com/Richmondlgf/audacity/blob/master/audacity-src/trunk/lib-src/portmixer/src/px_mixer.h

This is a follow up of the band aid bug:
Bug #⁠1306253 Rename master/headphone volume to gain.

[mixxxbot]

Commented by: daschuer
Date: 2015-04-06T09:34:03Z

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I have checked this for my Maya44 USB. It offers a single Hardware volume slider for the four output channel.
Using this for master volume will change the headphone loudness as well.

Since PortMixer is a generic wrapper it has the same issue, it offers only one master slider for each PA stream.

Not ideal, but in any case it would be great to allow to map the hardware slider from Midi and skin.

We may extend the solution to allow to map any hardware control.