Rename fallback-svg and fallback-png to fallback-image

- Merge fallback-svg and fallback-png into one fallback-image class.
- Move the vertical centering of PNG to MathTexvc and ext.math.js.
- Remove "img" from ext.math.css to reduce CSS specificity and make
  style easier to override by user stylesheets.
- Add a comment about the !important rules for centering in
  display mode.
- Only load ext.math.scripts in MathML mode.

Bug: 71955
Change-Id: If8ee1cf6453257a0a2f6aa186d4007954a8e5d8e