Force key frame on PLI

Also make the ReadCloser an Controllable allows to uncouple
the controller implementation from the encoder.
This is not needed for the 2 codec controller already implemented (openh264 and vpx)
but is more future proof in case it required for other codecs.

pkg/codec/codec.go

@@ -112,15 +112,37 @@ type VideoEncoderBuilder interface {
 	BuildVideoEncoder(r video.Reader, p prop.Media) (ReadCloser, error)
 }
 
-// ReadCloser is an io.ReadCloser with methods for rate limiting: SetBitRate and ForceKeyFrame
+// ReadCloser is an io.ReadCloser with a controller
 type ReadCloser interface {
 	Read() (b []byte, release func(), err error)
 	Close() error
+	Controllable
+}
+
+// EncoderController is the interface allowing to control the encoder behaviour after it's initialisation.
+// It will possibly have common control method in the future.
+// A controller can have optional methods represented by *Controller interfaces
+type EncoderController interface{}
+
+// Controllable is a interface representing a encoder which can be controlled
+// after it's initialisation with an EncoderController
+type Controllable interface {
+	Controller() EncoderController
+}
+
+// KeyFrameController is a interface representing an encoder that can be forced to produce key frame on demand
+type KeyFrameController interface {
+	EncoderController
+	// ForceKeyFrame forces the next frame to be a keyframe, aka intra-frame.
+	ForceKeyFrame() error
+}
+
+// BitRateController is a interface representing an encoder which can have a variable bit rate
+type BitRateController interface {
+	EncoderController
 	// SetBitRate sets current target bitrate, lower bitrate means smaller data will be transmitted
 	// but this also means that the quality will also be lower.
 	SetBitRate(int) error
-	// ForceKeyFrame forces the next frame to be a keyframe, aka intra-frame.
-	ForceKeyFrame() error
 }
 
 // BaseParams represents an codec's encoding properties