API to read ImageInput into ImageBuf

In the comments of #4225, @zavinator suggested:

> it would be great if the ImageInput class could integrate more seamlessly with ImageBuf, for instance by returning an ImageBuf instance for each subimage along with its associated metadata.

This is a great idea, and I can think of several possible forms for this to take:
1. An ImageBuf constructor that takes an open ImageInput* where it reads from the "current" subimage of the input into the IB. (IB already has a write() method that writes to an open ImageOutput that you pass it.)
2. An ImageInput::read_imagebuf() method a bit like read_image(), but returns (or reads into?) an ImageBuf rather than reading into a memory buffer.
3. One of the above two, but as a fully standalone utility functions rather than being a method of either ImageBuf or ImageInput.
4. Some kind of iterator that, given an ImageInput* and ImageBuf*, reads each successive subimage from the ImageInput into the IB. (Choice 1: it's just the iterator and you call next() to advance; choice 2: it's a total black box and the caller doesn't iterate, but rather, what you supply is a lambda that is called for each subimage?)

Or something else I haven't thought of?

Please feel free to continue the discussion in the comments to this issue.

Start by reading the ImageInput and ImageBuf APIs, especially ImageBuf::write() and ImageInput::read_image(), then review the alternatives and discussion referenced from #4225. Done requires an agreed API for moving one or more ImageInput subimages into ImageBuf instances with associated metadata, but the issue does not yet choose a design.

Written by the indexing model from the issue text.