Change receiveMTU from 1460 -> 1500

Raise the default ReceiveMTU to 1500, as same
as the default MTU setting of WebRTC. This is because
some senders, i.e. FFmpeg, may packetize RTP with 1460
payload + 12 RTP header + etc.

Fixes #2927

constants.go

@@ -6,8 +6,9 @@ package webrtc
 import "github.com/pion/dtls/v3"
 
 const (
-	// Equal to UDP MTU
-	receiveMTU = 1460
+	// default as the standard ethernet MTU
+	// can be overwritten with SettingEngine.SetReceiveMTU()
+	receiveMTU = 1500
 
 	// simulcastProbeCount is the amount of RTP Packets
 	// that handleUndeclaredSSRC will read and try to dispatch from