Fix #7 to remove the hardcoded iOvertoneVolume dependency and add the
abilty for a user to insert their own uniform parameters via a
user-draw-fn.  See tone.clj/overtone-volume fn.  Perhaps we can make
this a bit more abstract, but this is a start.

Move Overtone functions out of shader.clj and voltap.clj and put them
all in tone.clj.

Adjust demos and core.clj to reflect these changes.