Avoid Date object creation.

[kitschpatrol]

Very minor (20–30%) performance improvements by avoiding new Date() in fps-graph.ts and cubic-bezier-preview.ts.

const iterations = 10_000_000;

console.time("new Date().getTime()");
for (let i = 0; i < iterations; i++) {
  const t = new Date().getTime();
}
console.timeEnd("new Date().getTime()");

console.time("Date.now()");
for (let i = 0; i < iterations; i++) {
  const t = Date.now();
}
console.timeEnd("Date.now()");

// new Date().getTime(): 399.740966796875 ms
// Date.now(): 276.43994140625 ms

Just noticed it while working on #21.