AFP: add updated artwork, fix the bug where opening the plugin w/o samplebuffer glitches the wavegraph

Samplebuffer: use qualitysettings for interpolation (currently defaults to SINC_FASTEST on playback), also: in visualize, draw both channels instead of averaging them into one graph (otherwise, samplebuffers with counter-phase content show up as flatline)

include/SampleBuffer.h

@@ -42,6 +42,8 @@
 class QPainter;
 
 
+const f_cnt_t MARGIN = 4;
+
 class EXPORT SampleBuffer : public QObject, public sharedObject
 {
 	Q_OBJECT