Synapse's media repository does not preserve colourspace when thumbnailing

### Description

Wide gamut images get their colourspace ignored or removed when thumbnailed by Synapse, making the thumbnail look distinctly drab.

### Steps to reproduce

* Upload an image with wide gamut colour to Synapse (e.g. a JPEG in P3 colour space): https://matrix-client.matrix.org/_matrix/media/v3/download/matrix.org/qcexnobqNXHNFShayQgolvsy
* Observe that thumbnail calculated by the sending client is P3 and nice and vivid (when viewed on a wide gamut display): https://matrix-client.matrix.org/_matrix/media/v3/download/matrix.org/IqxsdtpAuPvBYfcbsyqcNTxF
* Observe that the thumbnail calculated by Synapse is drab and no longer wide gamut: https://matrix-client.matrix.org/_matrix/media/v3/thumbnail/matrix.org/qcexnobqNXHNFShayQgolvsy?width=1600&height=1200&method=scale. From a guess, it's just stripping off the colourspace to a default sRGB when thumbnailing it.

### Homeserver

matrix.org

### Synapse Version

1.101.0 (b=matrix-org-hotfixes,beff0a756e)

### Installation Method

Other (please mention below)

### Database

postgres

### Workers

Multiple workers

### Platform

Debian 10.13

### Configuration

_No response_

### Relevant log output

```shell
None
```

### Anything else that would be useful to know?

_No response_