Serve scaled images instead of original

Problem/Motivation

I'm grateful to have this at all, since core currently can't handle sizing at all, but it would be neat if it could serve a resized image instead of the original.

The current implementation serves the original image and uses the width and height attributes to scale it to the selected size.

This can cause performance issues with larger images being scaled down to smaller sizes, as the filesize can be much higher than if an appropriately-sized image was served.

Steps to reproduce

Upload a large image (say, a desktop wallpaper). Size it to be smaller than the original image. View the image being served and note that it is the original size.

Proposed resolution

Serve a scaled image.

Remaining tasks

Figure out how to do this.

User interface changes

None, unless you think this should be opt-in, in which case there'd need to be a way to turn it on and off.

API changes

None, I think.

Data model changes

None, I think.