Change to ppm format from png to get better performance

DemoPrograms/Demo_OpenCV_Draw_On_Webcam_Image.py

@@ -23,7 +23,7 @@ def main():
             break
 
         ret, frame = cap.read()
-        imgbytes=cv2.imencode('.png', frame)[1].tobytes()
+        imgbytes=cv2.imencode('.ppm', frame)[1].tobytes()
         if a_id:
             graph_elem.delete_figure(a_id)             # delete previous image
         a_id = graph_elem.draw_image(data=imgbytes, location=(0,0))    # draw new image
@@ -34,4 +34,4 @@ def main():
 
     window.close()
 
-main()
+main()