Media inititally loads too small in the timeline

### Steps to reproduce

Navigate to any room with media in it.
Observe how media first loads when not cached.

### Outcome

#### What did you expect?

Media should load with the aspec ratio calculated via the event's width and height, and maximize their size in the timeline.

#### What happened instead?

Media loads far too small, aspect ratio seems to be applied correctly, see image.

Media size in settings (default/large) does not seem to affect this behavior, it only gets the correct size when the image starts loading, the `` sets the correct size.

### Operating system

macOS, NixOS

### Browser information

Chrome Version 149.0.7827.200

### URL for webapp

_No response_

### Application version

Element version: 1.12.23-rc.0

### Homeserver

Synapse 1.155.0

### Will you send logs?

No

Reproduce the issue in a room containing uncached media and observe its initial timeline size. Trace the timeline media rendering and loading path, then verify that initial dimensions use the event width and height and that the media reaches its expected timeline size before loading completes.

Written by the indexing model from the issue text.