Butterfly ADST based hybrid transform

Refactor the 8x8 inverse hybrid transform. It is now consistent
with the new inverse DCT. Overall performance loss (due to the
use of this variant ADST, and the rounding errors in the butterfly
implementation) for std-hd is -0.02.

Fixed BUILD warning.

Devise a variant of the original ADST, which allows butterfly
computation structure. This new transform has kernel of the
form: sin((2k+1)*(2n+1) / (4N)). One of its butterfly structures
using floating-point multiplications was reported in Z. Wang,
"Fast algorithms for the discrete W transform and for the discrete
Fourier transform", IEEE Trans. on ASSP, 1984.

This patch includes the butterfly implementation of the inverse
ADST/DCT hybrid transform of dimension 8x8.

Change-Id: I3533cb715f749343a80b9087ce34b3e776d1581d