Debounce localstorage save (#328)

I profiled dragging and it looks like it takes ~3ms to save to localStorage a smallish scene and we're doing it twice per mousemove. Let's debounce so we don't pay that cost on every mouse move.

Stole the implementation from #220 which got reverted.

src/utils.ts

@@ -51,3 +51,14 @@ export function measureText(text: string, font: string) {
 
   return { width, height, baseline };
 }
+
+export function debounce<T extends any[]>(
+  fn: (...args: T) => void,
+  timeout: number
+) {
+  let handle = 0;
+  return (...args: T) => {
+    clearTimeout(handle);
+    handle = window.setTimeout(() => fn(...args), timeout);
+  };
+}