Correct on-screen kerning in math for non-mu units

The current math style should have no impact on \kern1mu. This is another
benefit of a9eb87a8.

Testcase:
\begin{align*}
 & \kern1em a\\
 & {\scriptscriptstyle \kern1em a}\\
 & \kern18mu a\\
 & {\scriptscriptstyle \kern18mu a}
\end{align*}

src/Length.cpp

@@ -302,7 +302,11 @@ int Length::inPixels(MetricsBase const & base) const
 {
 	FontInfo fi = base.font;
 	if (unit_ == Length::MU)
+		// mu is 1/18th of an em in the math symbol font
 		fi.setFamily(SYMBOL_FAMILY);
+	else
+		// Math style is only taken into account in the case of mu
+		fi.setStyle(LM_ST_TEXT);
 	return inPixels(base.textwidth, theFontMetrics(fi).em());
 }