Fix small glitch with \underXXXarrow and \overXXXarrow

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.