[TeX] \overset should prevent movable limits

\overset can create movable limits which does not match real LaTeX behavior.

E.g., \overset {\rightarrow}{\prod } creates <mover> <mo>&#x220F;</mo> <mo stretchy="false">&#x2192;</mo> </mover> which, if inline, will render as a superscript (as expected). Real LaTeX always renders the arrow above the product.