Improve matching of pdflatex font sizes to xelatex

The regular font weight had a gigantic difference between normal and fat
text. Whereas for the book font weight there was barely any difference
at all.

beamertheme-source/beamerfontthemeneo.dtx

@@ -124,11 +124,12 @@
       \neo@fontsave
     }
     \newcommand{\neo@fontbook}{
-      \def\bfseries@sf{mb}
+      \def\mdseries@sf{sl}
+      \def\bfseries@sf{medium}
       \neo@fontsave
     }
     \newcommand{\neo@fontregular}{
-      \def\mdseries@sf{m}
+      \def\mdseries@sf{sl}
       \def\bfseries@sf{sb}
       \neo@fontsave
     }