[XeTeX] Math accent bug

[Joel C. Salomon]

Big hats, little hats:

	% !TEX TS-program = xelatex
	% !TEX encoding = UTF-8
	\documentclass{minimal}
	
	\usepackage{unicode-math}
	\setmathfont{Cambria Math}
	
	\begin{document}\noindent
	Differently-sized accents for thin \& wide chars:
		$\hat\imath\hat w$,
	which is good \\
	---but the accent also gets wider to cover subscripts:
		$\hat x\hat x_x$,
	which is less good.\\
	Notice that only the width it altered, not the position;
	specifically, these are not the same:
		$\hat x^{xxxx}\hat{x^{xxxx}}$.
	\end{document}

There was an issue with the \vec & \dot accents where the position of
the accent shifted, making  $\dot x_x$  identical to  $\dot{x_x}$ ; I
was writing  $\dot x{}_x$  for a while.  That part of the problem was
fixed, but it seems some code still wants to glue the subscript to the
base and the \hat accent suffers.

—Joel Salomon