[fix] masked bounds calculation (#2197)

This PR fixes a bug where frames with children that have identical
dimensions would not be able to export as images.

When calculating masked page bounds, identical shapes would produce a
zero width/height masked page bounds.

### Change Type

- [x] `patch` — Bug fix

### Test Plan

1. Create a frame.
2. Create an image that is a child of the frame and the exact dimensions
of the frame (possibly using the console)
3. Export the image

### Release Notes

- Fix bug with getmaskedpagebounds calculation for identical parent /
child sizes

packages/editor/src/lib/editor/Editor.ts

@@ -3938,13 +3938,13 @@ export class Editor extends EventEmitter<TLEventMap> {
 		return this.store.createComputedCache<string, TLShape>('clipPathCache', (shape) => {
 			const pageMask = this._shapeMaskCache.get(shape.id)
 			if (!pageMask) return undefined
-			const pageTransform = this._shapePageTransformCache.get(shape.id)
-			if (!pageTransform) return undefined
-
 			if (pageMask.length === 0) {
 				return `polygon(0px 0px, 0px 0px, 0px 0px)`
 			}
 
+			const pageTransform = this._shapePageTransformCache.get(shape.id)
+			if (!pageTransform) return undefined
+
 			const localMask = Matrix2d.applyToPoints(Matrix2d.Inverse(pageTransform), pageMask)
 
 			return `polygon(${localMask.map((p) => `${p.x}px ${p.y}px`).join(',')})`
@@ -4040,7 +4040,13 @@ export class Editor extends EventEmitter<TLEventMap> {
 		if (!pageBounds) return
 		const pageMask = this._shapeMaskCache.get(shape)
 		if (pageMask) {
-			const intersection = intersectPolygonPolygon(pageMask, pageBounds.corners)
+			if (pageMask.length === 0) return undefined
+
+			const { corners } = pageBounds
+			if (corners.every((p, i) => Vec2d.Equals(p, pageMask[i]))) return pageBounds.clone()
+
+			// todo: find out why intersect polygon polygon for identical polygons produces zero w/h intersections
+			const intersection = intersectPolygonPolygon(pageMask, corners)
 			if (!intersection) return
 			return Box2d.FromPoints(intersection)
 		}