Add Alpha fading mode to media player timeline

Fading color (to black) is not useful for media with transparency; there fading of alpha is necessary; the fading mode allows to select fade color or fade alpha. Also the source control window had to be adapted, with a checkerboard to show the alpha effect. The cropping of the image in control window was also fixed.

src/MediaPlayer.cpp

@@ -72,6 +72,7 @@ MediaPlayer::MediaPlayer()
     video_filter_available_ = true;
     position_ = GST_CLOCK_TIME_NONE;
     loop_ = LoopMode::LOOP_REWIND;
+    fading_mode_ = FadingMode::FADING_COLOR;
 
     // default audio disabled
     audio_enabled_ = false;