Only render fg cells if there are changes

src/renderer/GridCache.ts

@@ -0,0 +1,20 @@
+export class GridCache<T> {
+  public cache: T[][];
+
+  public constructor() {
+    this.cache = [];
+  }
+
+  public resize(width: number, height: number) {
+    for (let x = 0; x < width; x++) {
+      if (this.cache.length <= x) {
+        this.cache.push([]);
+      }
+      for (let y = this.cache[x].length; y < height; y++) {
+        this.cache[x].push(null);
+      }
+      this.cache[x].length = height;
+    }
+    this.cache.length = width;
+  }
+}