webm broken on chromium/electron (frist frame overlay)

## Description

```json
"type": "m.room.message",
"sender": "@bubu:bubu1.eu",
"content": {
"msgtype": "m.video",
"body": "😎 (smiling face with sunglasses)",
"url": "mxc://bubu1.eu/KbfoXAMDFKtTCzAhbYjRAWjN",
"info": {
"mimetype": "video/webm",
"size": 115738,
"thumbnail_info": {
"mimetype": "image/png",
"size": 30983,
"h": 256,
"w": 256
},
"thumbnail_url": "mxc://bubu1.eu/JGEOxbqJcmImHDEmWVUxWcBC",
"h": 256,
"w": 256
}
}
```
Results in the following glitchy display in electron/chromium but plays fine in firefox (and nheko)


Looks like the initial frame gets never erased.

### Steps to reproduce

- send the above event via devtools
- view the event in element loaded on chromium

Expected: non glitchy playback### Version information

- **Platform**: web and desktop

For the web app:

- **Browser**: chromium 88.0.4324.96 or electron 11.2.0 (element-desktop archlinux build) or electron 10.2.0 (via [schildichat-desktop](https://github.com/SchildiChat/schildichat-desktop/releases) packages)
- **OS**: Arch Linux
- **URL**: Tested on 1.7.17 and 1.7.16