services/pipewire: consider device volume step when sending updates

Previously a hardcoded 0.0001 offset was used to determine if a volume
change was significant enough to send to a device, however some
devices have a much more granular step size, which caused future
volume updates to be blocked.

This change replaces the hardcoded offset with the volumeStep device
route property which should be large enough for the device to work with.

Fixes #279

src/services/pipewire/node.hpp

@@ -158,6 +158,7 @@ struct PwVolumeProps {
 	QVector<PwAudioChannel::Enum> channels;
 	QVector<float> volumes;
 	bool mute = false;
+	float volumeStep = -1;
 
 	static PwVolumeProps parseSpaPod(const spa_pod* param);
 };
@@ -214,6 +215,7 @@ private:
 	QVector<float> mServerVolumes;
 	QVector<float> mDeviceVolumes;
 	QVector<float> waitingVolumes;
+	float volumeStep = -1;
 	PwNode* node;
 };