Fix memory leak

https://github.com/mifi/editly/issues/53#issuecomment-675177499

sources/fabricFrameSource.js

@@ -32,7 +32,7 @@ async function renderFabricCanvas(canvas) {
   canvas.renderAll();
   const rgba = fabricCanvasToRgba(canvas);
   canvas.clear();
-  // canvas.dispose();
+  canvas.dispose();
   return rgba;
 }