API: relative POST player/volume seems to be broken

[2opremio]

I am using the Alsa mixer with the following configuration:

audio_device: default # ALSA audio device to use for playback
mixer_device: default # ALSA mixer device for volume synchronization 
mixer_control_name: Speaker # ALSA mixer control name for volume synchronization
volume_steps: 100 # Volume steps count
initial_volume: 100 # Initial volume in steps (not applied to the mixer device)
external_volume: false # Whether volume is controlled externally 

I am also using the POST player/volume endpoint to increase/decrease the volume in a relative manner:

To increase:

{
	"volume": 1,
	"relative": true
}

To decrease:

{
	"volume": -1,
	"relative": true
}

However, the volume only increases 1 step up or down (it doesn't continue increasing or decreasing upon multiple API calls).

Also, I get lots of this spurious (and seemly incorrect) warnings:

WARN[0015] error while handling alsa mixer events. (Success) 
WARN[0015] error while handling alsa mixer events. (Success) 
WARN[0015] error while handling alsa mixer events. (Success) 
WARN[0015] error while handling alsa mixer events. (Success) 

[2opremio]

BTW, setting the volume from an external Spotify client does seem to change the Alsa mixer volume correctly.

[2opremio]

Also, this is on 0.1.2 (I haven't checked other versions)

[2opremio]

If you turn down the volume very slowly (approx one change for second) then it works. This makes it unusable though.

I bet is that 4ad9ae1 caused this (CC @aykevl )

It's worth noting that if you you request volume changes faster than 1 per second, the volume doesn't update at all.

I think that the mixer update frequency should be independent of the frequency we send updates back to Spotify.

[aykevl]

I can confirm this issue with the PulseAudio output so it's not related to the ALSA mixer.

[aykevl]

Wrote a fix: #144
This fix actually simplifies the code because there's no real reason why the volume needs to be kept outside the player state.