[Image Redux] Use natural dimensions in AccessibilityRenderObject::computeIsIgnored()

https://bugs.webkit.org/show_bug.cgi?id=326354

Reviewed by Darin Adler.

Use natural dimensions rather than RenderImage::imageSizeAsRendered() in AccessibilityRenderObject::computeIsIgnored()
to more accurately do what the comment states the intent is:

  "check whether rendered image was stretched from one-dimensional file image"

Also adds an explicit check for whether there is an image at all to maintain the behavior expected by layout tests.

* Source/WebCore/accessibility/AccessibilityRenderObject.cpp:

Canonical link: https://commits.webkit.org/322768@main