rewriter: add vp8 keyframe detection function

Relates to #100

pkg/conference/subscription/rewriter/vp8.go

@@ -0,0 +1,29 @@
+package rewriter
+
+import (
+	"github.com/pion/rtp"
+	"github.com/pion/rtp/codecs"
+)
+
+// Determines if a given packet contains a VP8 keyframe.
+func IsVP8Keyframe(packet rtp.Packet) bool {
+	// First try to parse the packet as a VP8 packet.
+	vp8Packet := codecs.VP8Packet{}
+
+	payload, err := vp8Packet.Unmarshal(packet.Payload)
+	if err != nil {
+		return false
+	}
+
+	// At this point we know that we're dealing with a VP8 packet
+	// and we have a so-called "VP8 Payload Descriptor" that Pion
+	// parses into the `vp8Packet` structure. This is not to be
+	// confused with the "VP8 Payload Header", one bit of which
+	// we're parsing here. The P bit is set to 0 for the key frames.
+	Pbit := (payload[0] & 0x01)
+
+	// We also must check that the S bit from the VP8 Payload Descriptor
+	// is set to 1 as S denotes a start of a new VP8 partition. Typically
+	// key frames have it set to 1.
+	return vp8Packet.S == 1 && Pbit == 0
+}