Find any images that are in Form XObjects in the container

    The container may be a page, or a parent Form XObject.

def _find_form_xobject_images(pdf, container, contentsinfo):
    """Find any images that are in Form XObjects in the container

    The container may be a page, or a parent Form XObject.

    """
    if '/Resources' not in container:
        return
    resources = container['/Resources']
    if '/XObject' not in resources:
        return
    xobjs = resources['/XObject'].as_dict()
    for xobj in xobjs:
        candidate = xobjs[xobj]
        if candidate['/Subtype'] != '/Form':
            continue

        form_xobject = candidate
        for settings in contentsinfo.xobject_settings:
            if settings.name != xobj:
                continue

            # Find images once for each time this Form XObject is drawn.
            # This could be optimized to cache the multiple drawing events
            # but in practice both Form XObjects and multiple drawing of the
            # same object are both very rare.
            ctm_shorthand = settings.shorthand
            yield from _process_content_streams(
                pdf=pdf, container=form_xobject, shorthand=ctm_shorthand
            )