video: don't blacken frame if not needed

The function yuv422_clear_to_black is called up to 3 times per frame.
Remove the superfluous calls. This function only does useful in a
conference, when there can be blank space in a frame.

Change-Id: I64588adba59daaf3677bad419a519432eec41dff
Reviewed-by: Sébastien Blin <sebastien.blin@savoirfairelinux.com>

src/media/media_encoder.cpp

@@ -273,11 +273,6 @@ int
 MediaEncoder::encode(VideoFrame& input, bool is_keyframe,
                      int64_t frame_number)
 {
-    /* Prepare a frame suitable to our encoder frame format,
-     * keeping also the input aspect ratio.
-     */
-    yuv422_clear_to_black(scaledFrame_); // to fill blank space left by the "keep aspect"
-
     scaler_.scale_with_aspect(input, scaledFrame_);
 
     auto frame = scaledFrame_.pointer();

src/media/video/video_mixer.cpp

@@ -189,8 +189,6 @@ VideoMixer::setDimensions(int width, int height)
 
     // cleanup the previous frame to have a nice copy in rendering method
     std::shared_ptr<VideoFrame> previous_p(obtainLastFrame());
-    if (previous_p)
-        yuv422_clear_to_black(*previous_p);
 
     start_sink();
 }