An SVG shown as an image that could not be read (text that isn't XML,
an SVG without a size) still took one of the 256 nested SVG images an
SVG may show, so an image after 256 broken ones was left out. Only
images that decode now count toward that limit; a failed image gives
its place back.
Every nested SVG image read, decoded or not, counts toward new limits
of 1,024 reads and 512 MB of text per import, so a broken image shown
many times through <use> is still read a limited number of times. Past
them, nested SVG images are left out with a warning.
A nested SVG image with text that usvg then refuses no longer warns
that its text isn't imported.