Normal maps not correctly handled in CompressedImageSaver

CompressedImageSaver (post https://github.com/bevyengine/bevy/pull/23567?) doesn't handle normals maps properly, making this issue on behalf of @JMS55 so we don't forget it ([discord thread](https://discord.com/channels/691052431525675048/1487116765522694305/1500895319314010214)).

> Jasmine (JMS55) — 02:16
Hmm good point. You need to set is_srgb: false in the meta file, but iirc cwfitzgerald there's no way of telling ctt something's a normal map, right?
Because iirc you want to use a different filter than lancoz for normal map mips?
cwfitzgerald — 02:17
There nothing to be done on ctt's side, the user can adjust color space and if you want to adjust downscale parameters I guess
I don't know anything in particular about downscaling normal maps
Jasmine (JMS55) — 02:18
Yeah I think Bevy's usage is actually wrong
There's no way to garuntee that the normal is actually a normalized unit vector after compressing