When displaying SVGs inside a navigable directly, we want to display
them in the size specified by their width and height attributes instead
of stretching them to the viewport as layout normally would.
However, when doing so, we need to actually check that the SVG we are
laying out is indeed directly inside the navigable. Otherwise we just
blindly overwrite whatever content sizes have been calculated by layout
code for e.g. SVG elements inlined somewhere in an HTML document.
Due to the way this was written originally, the bug was not very
noticable. The code overwrote the content width/height with the computed
width/height, which was often still correct in the sense that those two
had the same value. However, content size may also be the result of
{min,max}-{width,height} constraints, which can make it differ from the
computed values.
This bug was likely a regression introduced in
f5e01192cc.
Previously if we encountered any attributes with a namespace other than
`xml` or `xmlns`, we treated it as a parse error. Now, allow it as long
as it's been declared in the current context.
We also handle errors more gracefully - instead of exploding if setting
the namespace fails, treat it as an error and carry on.
