SVG images should not be loaded when MathML is used

- Use span with a background-image instead of <img> for the fallback SVG so
  that they don't load in Gecko.
- Make the image fallback a span inline-block so that it can have a dimension.

Bug: 71929
Change-Id: I7f820cd5766db1fece452ebfc72915a55e42c82e