fix: tank widget looks mangled when background is set transparent, due to how transparent pixels blend over opaque pixels

Author: cjenkscybercom

app/rtplot/src/main/java/org/csstudio/javafx/rtplot/RTTank.java

@@ -92,6 +92,9 @@ public void refreshPlotPart(final PlotPart plotPart)
         if (image != null)
             synchronized (image)
             {
+                // Clear the canvas for e.g. transparent pixels (otherwise image appears drawn over previous)
+                gc.clearRect(0, 0, gc.getCanvas().getWidth(), gc.getCanvas().getHeight());
+                // Draw the updated image
                 gc.drawImage(image, 0, 0);
             }
     };