Add fog of war example (#4025)

This PR adds a cute fog of war example.

![Kapture 2024-06-26 at 20 47
37](https://github.com/tldraw/tldraw/assets/23072548/b426776f-b027-419e-9770-29f5c7ab2563)

### Change Type

- [x] `docs` 
- [x] `improvement` 

### Release Notes

- Adds fog of war example.

apps/examples/src/examples/fog-of-war/FogOfWarExample.tsx

@@ -0,0 +1,106 @@
+import { useEffect, useRef } from 'react'
+import { Box, TLComponents, Tldraw, Vec, useEditor, useReactor } from 'tldraw'
+import 'tldraw/tldraw.css'
+
+const CELL_SIZE = 32
+const COUNT = 100
+
+const boxes: Box[][] = []
+const cells: boolean[][] = []
+for (let i = 0; i < COUNT; i++) {
+	cells[i] = []
+	boxes[i] = []
+	for (let j = 0; j < COUNT; j++) {
+		cells[i].push(false)
+		boxes[i].push(
+			new Box((i - COUNT / 2) * CELL_SIZE, (j - COUNT / 2) * CELL_SIZE, CELL_SIZE, CELL_SIZE)
+		)
+	}
+}
+
+export function Fog() {
+	const rCanvas = useRef<HTMLCanvasElement>(null)
+	const rVisibility = useRef<boolean[][]>(cells)
+	const editor = useEditor()
+
+	useEffect(() => {
+		const cvs = rCanvas.current!
+		const rect = cvs.getBoundingClientRect()
+		cvs.width = rect.width
+		cvs.height = rect.height
+	}, [editor])
+
+	useReactor(
+		'update fog',
+		() => {
+			const cells = rVisibility.current
+			const shapes = editor.getCurrentPageShapes()
+			for (const shape of shapes) {
+				const point = editor.getShapePageBounds(shape)!.point
+				const geometry = editor.getShapeGeometry(shape)
+				for (let i = 0; i < boxes.length; i++) {
+					for (let j = 0; j < boxes[i].length; j++) {
+						const box = boxes[i][j]
+						box.translate(Vec.Neg(point))
+						if (geometry.bounds.collides(box)) {
+							cells[i][j] = true
+						}
+						box.translate(point)
+					}
+				}
+			}
+			const cvs = rCanvas.current!
+			const ctx = cvs.getContext('2d')!
+
+			ctx.resetTransform()
+			const camera = editor.getCamera()
+
+			ctx.clearRect(0, 0, cvs.width, cvs.height)
+			ctx.fillStyle = 'rgba(0,0,0,0.9)'
+			ctx.fillRect(0, 0, cvs.width, cvs.height)
+
+			ctx.translate(100, 100)
+			ctx.scale(camera.z, camera.z)
+			ctx.translate(camera.x, camera.y)
+
+			for (let i = 0; i < boxes.length; i++) {
+				for (let j = 0; j < boxes[i].length; j++) {
+					if (!cells[i][j]) continue
+					const box = boxes[i][j]
+					ctx.filter = 'drop-shadow(100px)'
+					ctx.clearRect(box.x, box.y, box.width, box.height)
+				}
+			}
+		},
+		[editor]
+	)
+
+	return (
+		<canvas
+			ref={rCanvas}
+			style={{
+				zIndex: 999999,
+				position: 'absolute',
+				top: -100,
+				left: -100,
+				width: 'calc(100% + 200px)',
+				height: 'calc(100% + 200px)',
+				WebkitFilter: 'blur(15px)',
+				filter: 'blur(15px)',
+				pointerEvents: 'none',
+			}}
+		/>
+	)
+}
+
+const components: TLComponents = {
+	InFrontOfTheCanvas: Fog,
+}
+
+export default function BasicExample() {
+	return (
+		<div className="tldraw__editor">
+			<Tldraw persistenceKey="example" components={components} />
+		</div>
+	)
+}

apps/examples/src/examples/fog-of-war/README.md

@@ -0,0 +1,10 @@
+---
+title: Fog of war
+component: ./FogOfWarExample.tsx
+category: basic
+keywords: [ui, fog, overlay]
+---
+
+---
+
+This example shows how you might keep an HTML canvas in sync with canvas content. It implements a simple "fog of war" effect, where the canvas is covered by a black overlay that can be cleared by drawing shapes on the canvas.