Update modules/ext.math.css

Currently, the PNG fallback is hidden in the MathML mode and the MathML & SVG
are respectively hidden and visible by default; the visibility of the
SVG/MathML output is only changed for Gecko. This does not work well for
IE < 9, which does not support SVG. This commit tries to improve the selection
of PNG, SVG or MathML according to the browser capatibility.

Change-Id: Ie9b0fdb66c51fbf6be98df092ce4de29fbba6068