- An image alone in its paragraph (but not a figure) is now
rendered as an independent image, with an `alt` attribute
if a description is supplied.
- An inline image that is not alone in its paragraph will
be rendered, as before, using a substitution.
Such an image cannot have a "center", "left", or
"right" alignment, so the classes `align-center`,
`align-left`, or `align-right` are ignored.
However, `align-top`, `align-middle`, `align-bottom`
will generate a corresponding `align` attribute.
Closes#6948.
