Fix small glitch with \underXXXarrow and \overXXXarrow

When the argument is long enough, the arrow in the on-screen representation
was not covering the first or the last glyphs. For example, using
\overrightarrow{abcdefghijklmnopqrstuvwxyz}
the 'a' was drawn outside the covering arrow.

src/mathed/MathSupport.cpp

@@ -235,7 +235,7 @@ double const arrow[] = {
 	0.0500, 0.7500, 0.2000, 0.6000, 0.3500, 0.3500,
 	0.5000, 0.0500, 0.6500, 0.3500, 0.8000, 0.6000,
 	0.9500, 0.7500,
-	3, 0.5000, 0.1500, 0.5000, 0.9500,
+	3, 0.5000, 0.1500, 0.5000, 1.0000,
 	0
 };