Disable MediaEngine Copy by Default

If an API is shared between PeerConnections they would use the same
MediaEngine. A MediaEngine contains negotiated PayloadTypes so if the
PeerConnections were answering you would end up in invalid states.

Add DisableMediaEngineCopy to SettingEngine in case user needs old
behavior.

Resolves #1662

settingengine.go

@@ -59,6 +59,7 @@ type SettingEngine struct {
 	LoggerFactory                             logging.LoggerFactory
 	iceTCPMux                                 ice.TCPMux
 	iceProxyDialer                            proxy.Dialer
+	disableMediaEngineCopy                    bool
 }
 
 // DetachDataChannels enables detaching data channels. When enabled
@@ -255,3 +256,10 @@ func (e *SettingEngine) SetICETCPMux(tcpMux ice.TCPMux) {
 func (e *SettingEngine) SetICEProxyDialer(d proxy.Dialer) {
 	e.iceProxyDialer = d
 }
+
+// DisableMediaEngineCopy stops the MediaEngine from being copied. This allows a user to modify
+// the MediaEngine after the PeerConnection has been constructed. This is useful if you wish to
+// modify codecs after signaling. Make sure not to share MediaEngines between PeerConnections.
+func (e *SettingEngine) DisableMediaEngineCopy(isDisabled bool) {
+	e.disableMediaEngineCopy = isDisabled
+}