rebased.

This patch includes 16x16 butterfly inverse ADST/DCT hybrid
transform. It uses the variant ADST of kernel
    sin((2k+1)*(2n+1)/4N),
which allows a butterfly implementation.

The coding gains as compared to DCT 16x16 are about 0.1% for
both derf and std-hd. It is noteworthy that for std-hd sets
many sequences gains about 0.5%, some 0.2%. There are also few
points that provides -1% to -3% performance. Hence the average
goes to about 0.1%.

Change-Id: Ie80ac84cf403390f6e5d282caa58723739e5ec17