Make accents zero width (#1033)

* Make accents zero width

Make the width of an accented character equal to the width of the character,
by making accents zero width.  In particular, fixes #1028 (`\ddot\imath`).

This involved reworking all of the accent offset machinery, in particular
skew and accent-hungarian.  A bit cleaner now.

Also added support for the new width character metrics in symbolNode.

* Update AccentsText test

* Fix comment

test/screenshotter/ss_data.yaml

@@ -14,8 +14,8 @@
 Accents: \vec{A}\vec{x}\vec x^2\vec{x}_2^2\vec{A}^2\vec{xA}^2
 AccentsText: |
     \begin{array}{l}
-    \text{\H{e}}\\
-    \text{\H{X}}\\
+    \text{\H{e}} & \text{\'\i} \\
+    \text{\H{X}} & \text{\"\i} \\
     \end{array}
 Aligned: |
     \begin{aligned}