bug(cdk-experimetal/scrolling): Every item of input array is rendered initially

### Is this a regression?

- [ ] Yes, this behavior used to work in the previous version

### The previous version in which this bug was not present was

_No response_

### Description

Initially, before any scrolling occurs; every item of `items` passed to `cdkVirtualFor` is rendered. After scrolling down for a bit (till `translateY` of `.cdk-virtual-scroll-content-wrapper` changes) and scrolling up (even just a bit) the list of rendered elements sharing to only visible elements.

### Reproduction

Could not create stackblitz with `cdk-experimental` nor `angular` v13. Here is an example of what it does in my fairly rudimentary case: https://youtu.be/ILHPaMk2hZU

related code:
```html

```
-----

Steps to reproduce:
1. Create a container and any item template. Fixed height for simplicity is perfect
```

```
2. Populate `items` array with 1000 elements
3. View in elements panel, that there is the same number of `div`s as there is of `items`.

### Expected Behavior

I expected only needed elements to be rendered.

### Actual Behavior

Every element of my array is rendered making `cdk-virtual-scroll-viewport` do nothing!

### Environment

Angular CLI: 13.0.3
Node: 14.15.4
Package Manager: npm 7.20.6
OS: win32 x64

Angular: 13.0.2
... animations, cdk, cdk-experimental, common, compiler
... compiler-cli, core, forms, language-service, material
... platform-browser, platform-browser-dynamic, router
... service-worker

Package Version
---------------------------------------------------------
@angular-devkit/architect 0.1300.3
@angular-devkit/build-angular 13.0.3
@angular-devkit/core 13.0.3
@angular-devkit/schematics 13.0.3
@angular/cli 13.0.3
@angular/google-maps 12.1.3
@schematics/angular 13.0.3
rxjs 7.4.0
typescript 4.4.4

Start by reproducing the provided cdkVirtualFor example with 1,000 fixed-height items and inspect the rendered elements before and after scrolling. Trace the cdkVirtualFor and autosize behavior to determine why the initial render includes every item; done means only the needed visible elements are rendered from the start.

Written by the indexing model from the issue text.