Bug 1648064 - Switch DOM images to work like CSS images for the purposes of printing. r=tnikkel,smaug

Make them perform the image load (if needed), instead of copying the
image requests from the original document.

This is needed for CSS for stuff like:

@media print {
  #foo::before {
    content: url(bar.png);
  }
}

And so on. For images, we should do this as well. Nothing prevents you
from doing:

  <picture>
    <source srcset="print.png" media="print">
    <source srcset="screen.png" media="not print">
    <img>
  </picture>

And that should in theory work. It works after this patch, and I added a
test for that.

This patch is a bit bigger than I'd like, but I didn't find a more
reasonable way to split it up.

Making static docs able to do image loads is most of the patch and is
mostly straight-forward. This allows to remove the hacky "change the
loading document" thing that CSS images do, which is just working around
the CSP of the print document.

I need to enable background colors in printpreview_helper so as to be
able to have a reference page for all the different image types.

Differential Revision: https://phabricator.services.mozilla.com/D81779

dom/html/HTMLInputElement.cpp

@@ -1088,9 +1088,6 @@ nsresult HTMLInputElement::Clone(dom::NodeInfo* aNodeInfo,
       }
       break;
     case VALUE_MODE_DEFAULT:
-      if (mType == NS_FORM_INPUT_IMAGE && it->OwnerDoc()->IsStaticDocument()) {
-        CreateStaticImageClone(it);
-      }
       break;
   }