More information on how EXIF image sizing actually works

From FF90 (and similar vintage browsers) browsers respect the image size information in EXIF for working out the intrinsic sizes of an image. This was added for Client Hint HTTP headers - it allows a server to return a low-res placeholder image of the correct size for layout, and then later supply a higher resolution image. Detailed explainer on that here: https://github.com/eeeps/exif-intrinsic-sizing-explainer

Anyway, the existence of this feature was added in #6798. We will also add some information once Client Hints are accepted in the standard.

However what that is likely to miss is the detail on exactly what EXIF fields must be set. There is some discussion of that in whatwg/html#5574 - sounds like we need all four of PixelXDimension, PixelYDimension, ResolutionX, and ResolutionY, ... and ResolutionUnit.

Further links:
- https://github.com/eeeps/exif-intrinsic-sizing-explainer
- https://github.com/WICG/responsive-image-client-hints/issues/14#issuecomment-865413309
- #6240
- #6835