The DST2 is implemented by input alternate sign-flip, followed
by DCT, followed by output reversal.
Results are roughly the same, but it should be easier to optimize
the DST2.
[Interestingly a mtrix multuiply implementation is about 0.1%
better].

Change-Id: If9ae5fdba87767fb0e6c163a62b77ee66a8d3afc