Changes from MusiXTeX 120 to 121:
+ adjusted the offsets and/or vertical positions for some tremolos
+ moved \lastbarpos\lin@pos after calls to \Writ@meters
to correct \centerbar positioning
+ copied the definition of \Hpause from musixlit.tex
+ introduced \Changeclefs, \cHpause and \centerHpause
+ replaced \smalltype\it by \txtfont in octave clef commands
+ moved definitions of \txtfont into \comput@fonts
+ added support for piano-pedal bracket notation