Follow the HTML specification and take into account that the HTML
`<image>/<source>` element inserting/removal steps should only be
counted as relevant mutations for `<image>` element if the parent of the
inclusive ancestor that was inserted/removed is the parent `<picture>`
element.
See <https://html.spec.whatwg.org/multipage/#relevant-mutations>.
Testing: Improvements in the following tests
-
html/semantics/embedded-content/the-img-element/relevant-mutations.html
Signed-off-by: Andrei Volykhin <andrei.volykhin@gmail.com>