Do not create receiver for ealy media in offerer

For offerer, if the remote side sends early media
before the remote description (answer) is received,
the undeclared SSRC processor can create a receiver
and that receiver could be left dangling as
transceiver `mid` is not updated from remote
description answer.

Still leaving the simulcast probe path and only
avoiding creating a receiver for non-simulcast path.

Add a flag `handleUndeclaredSSRCWithoutAnswer` to control handling
of early media without SDP answer for non-simulcast tracks.
The default behaviour is to not process early media without SDP answer.

settingengine.go

@@ -109,6 +109,7 @@ type SettingEngine struct {
 	fireOnTrackBeforeFirstRTP                 bool
 	disableCloseByDTLS                        bool
 	dataChannelBlockWrite                     bool
+	handleUndeclaredSSRCWithoutAnswer         bool
 }
 
 func (e *SettingEngine) getSCTPMaxMessageSize() uint32 {
@@ -570,3 +571,9 @@ func (e *SettingEngine) SetFireOnTrackBeforeFirstRTP(fireOnTrackBeforeFirstRTP b
 func (e *SettingEngine) DisableCloseByDTLS(isEnabled bool) {
 	e.disableCloseByDTLS = isEnabled
 }
+
+// SetHandleUndeclaredSSRCWithoutAnswer controls if an SDP answer is required for
+// processing early media of non-simulcast tracks.
+func (e *SettingEngine) SetHandleUndeclaredSSRCWithoutAnswer(handleUndeclaredSSRCWithoutAnswer bool) {
+	e.handleUndeclaredSSRCWithoutAnswer = handleUndeclaredSSRCWithoutAnswer
+}