Revert "video: don't blacken frame if not needed"

This reverts commit c4b580a2.

Change-Id: If4a00bde338aa657c482ae18cc59902ed1be593c

src/media/media_encoder.cpp

@@ -273,6 +273,11 @@ 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

@@ -189,6 +189,8 @@ 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();
 }