Slow addPage

ctx.addPage() very slow if some rects already drawed

## Issue
ctx.addPage() very slow if some rects already drawed. If no any drawed ctx.addPage() is quick

## Steps to Reproduce
```js
const { createCanvas } = require('canvas');

const width = 420;
const height = 297;
const ctx = createCanvas(width, height, 'pdf').getContext('2d');

const draw = () => {
console.time('draw');

for (let i = 0; i < 10000; ++i) {
const r1 = Math.random();
const r2 = Math.random();

ctx.strokeStyle = ctx.fillStyle = `rgba(${Math.floor(r1 * 255)}, ${Math.floor(r2 * 255)}, ${Math.floor(r1 * 255)}, ${r2})`;
ctx.fillRect(r1 * width, r2 * height, r1, r2);
ctx.strokeRect(r1 * width, r2 * height, r1, r2);
}

console.timeEnd('draw');
};

const addPage = () => {
console.time('addPage');

ctx.addPage();

console.timeEnd('addPage');
};

for (let i = 0; i < 3; ++i) {
draw();
addPage();
addPage();
addPage();
console.log('');
}
```

Results:
```
draw: 71.481ms
addPage: 538.003ms
addPage: 0.125ms
addPage: 0.068ms

draw: 59.522ms
addPage: 535.811ms
addPage: 0.086ms
addPage: 0.123ms

draw: 59.446ms
addPage: 538.068ms
addPage: 0.083ms
addPage: 0.127ms
```

## Your Environment
* Version of node-canvas: 2.2.0
* Environment: node 11.6.0 on Win10 Home