LaTeXML: prevent automatic rerendering of SVG

MathMathML::renderingRequired called the getSvg
function in MathLaTeXML that caused an automatic
rerendering of the SVG image if it was not cached.
That this rendering is triggered from the function
renderingRequired is contra intutive and should be
avoided.

Change-Id: Ifa14f50193a05b3058624a18974c56897fcec09e