feat: add marker bit when packetizing opus.

incase of silence opus rtp payload sets the marker bit after a talking spurt.
usually the data in silence pacekts are empty, just use 1-2 bytes for header.

fixes #125

src/packet/opus.rs

@@ -23,8 +23,12 @@ impl Packetizer for OpusPacketizer {
     }
 
     fn is_marker(&mut self, data: &[u8], previous: Option<&[u8]>, last: bool) -> bool {
-        // TODO: dtx
-        false
+        match previous {
+            // silence packets are usually empty with 1-2 bytes header
+            Some(previous) => previous.len() < 3 && data.len() >= 3,
+            // first packet can always be considered as start of talk spurt
+            None => true,
+        }
     }
 }