fix x-fades (part one)

fix x-fades (part one)

The data from the lower layer(s) was not faded out because
the reversed gain curve was incorrect because ControlList:add()
inserts anchor points.

a call to reverse_curve() for a linear fade produced:
 INPUT: [when,val]  {[0.0, 0.0], [300.0, 1.0]}
 OUTPUT: (reversed) {[0.0, 0.0], [0.0, 1.0], [1.0, 0,0], [300.0, 0.0]}

solution: use fast_simple_add() instead.

git-svn-id: svn://localhost/ardour2/branches/3.0@13572 d708f5d6-7413-0410-9779-e7cbd77b26cf