Fix flickering on board redraw

Turns out == operator can't be used to compare two JS objects. Implement
a custom method for determining piece equality.

frontend/visuals.js

@@ -154,6 +154,12 @@ class PieceVis {
   }
 }
 
+function pieceEqual(pieceA, pieceB) {
+  return (
+    pieceA.color === pieceB.color && pieceA.piece_type === pieceB.piece_type
+  );
+}
+
 export class ConfigVis {
   constructor(configuration) {
     this.configuration = configuration;
@@ -162,7 +168,7 @@ export class ConfigVis {
   draw(canvas) {
     for (let [position, piece] of this.configuration) {
       if (this.piecesVis.has(position)) {
-        if (this.piecesVis.get(position).piece == piece) {
+        if (pieceEqual(this.piecesVis.get(position).piece, piece)) {
           continue;
         }
       }