fix volume reset when unmuting by clicking the slider

[zWolfrost]

Fixes #24

This happened because when "set_muted" is called to unmute a meter, it checks if the volume is 0% on both sides and resets them to 100% if it is.

However, the volume it got was under some kind of latency and, when unmuting by clicking the slider, "set_muted" still thinks the volume is 0% (from both sides), thus resetting the volume to 100% anyway.

By setting the variable "zero_volume_reset" i have made sure that the volume is not reset when unmuting by clicking the slider, but only when clicking the button directly.