#548 OpalMixer doesn't mix sound from IVR

[Sysolyatin Pavel]

One mixer node. 2 SIP calls.
When I create IVR call to this node, nobody hear sound from this IVR call.

[Sysolyatin Pavel]

OpalMixerMediaStream::Start() never called for streams from IVR. And no input streams attached to mixer from IVR calls.
If I remove virtual PBoolean Start() at all and add:

if (!IsPaused() && !m_node->AttachStream(this))
   return false;

to OpalMixerMediaStream::Open() everything works fine.