[graph] take screenshot of canvases in proper order

We should hide edges under nodes, this was the other way around before
the change.

src/Gargantext/Hooks/Sigmax/Sigma.js

@@ -194,10 +194,14 @@ function takeScreenshot(sigma) {
   let c = sigma.renderers[0].container;
   let edges = c.getElementsByClassName('sigma-edges')[0];
   let scene = c.getElementsByClassName('sigma-scene')[0];
-  let sceneCtx = scene.getContext('2d');
-  sceneCtx.globalAlpha = 1;
-  sceneCtx.drawImage(edges, 0, 0);
-  return scene.toDataURL('image/png');
+  // let sceneCtx = scene.getContext('2d');
+  // sceneCtx.globalAlpha = 1;
+  // sceneCtx.drawImage(edges, 0, 0);
+  // return scene.toDataURL('image/png');
+  let edgesCtx = edges.getContext('2d');
+  edgesCtx.globalAlpha = 1;
+  edgesCtx.drawImage(scene, 0, 0);
+  return edges.toDataURL('image/png');
 }
 
 function getEdges(sigma) {