Fix collaborator size with zoom (#3563)

Fix a bug with the size of collaborators.

### Change Type

- [x] `sdk` — Changes the tldraw SDK
- [x] `bugfix` — Bug fix

packages/tldraw/src/lib/ui/components/Minimap/MinimapManager.ts

@@ -134,9 +134,7 @@ export class MinimapManager {
 	}
 
 	@computed getZoom() {
-		const cpb = this.getCanvasPageBounds()
-		const vp = this.editor.getViewportPageBounds()
-		return cpb.width / vp.width
+		return this.getCanvasPageBounds().width / this.getCanvasScreenBounds().width
 	}
 
 	@computed getCanvasPageBoundsArray() {