Use spans for displaystyle fallback images

Previously, we were using div containers to SVG fallback images to
mimic the MathML  behaviour. This had unintended side
effects as reported in T182041.

This change uses span containers for both display and inline math
fallback images.

Bug: T182041
Change-Id: I1d563f9f3cba8a10af2665518be6c672de174333
1 file changed