Add ImageFormat::Bgra8

[sagudev]

Vello can handle this easily with textureSample which automagically makes it RGBA.
Actually vello uses textureLoad and atlas texture, so we would need to implement this as in vello_cpu, preferably with swaping on image cmd (this should be just bgra vs rgba swizle).

[DJMcNab]

I'd like to see a PR properly integrating this in Vello Classic, to make sure it doesn't cause us too much pain.

I think broadly we're in favour of this

[sagudev]

I'd like to see a PR properly integrating this in Vello Classic, to make sure it doesn't cause us too much pain.

Done: linebender/vello#1173

[DJMcNab]

We'll probably discuss this in renderer office hours. I can't see why it should be avoided, and the impl in Vello makes sense to me. (Obviously longer-term, the story here will need to change).

This does need a changelog entry. I'm of two minds as to whether we should mark it as a breaking change.

[sagudev]

This does need a changelog entry. I'm of two minds as to whether we should mark it as a breaking change.

This is not breaking change because of #[non_exhaustive], but it's intended to ship with rest of breaking changes.

[DJMcNab]

We discussed this in #office hours > 2025-08-28, and have decided that we are able to support this.

This still needs a CHANGELOG entry, but then I'll be happy to approve this.
Thanks!