libvpx

Replacing the 8x8 DCT with 8x8 ADST/DCT for I8x8

Fixed the code review comments.

Under the htrans8x8 experiment the 8X8 DCT in the
I8X8 mode is replaced with a combination of 8X8 ADST and
DCT.

Overall coding gains with the htrans8x8 experiment are:
derf:   0.486
std-hd: 1.040
hd:     1.063
yt:     0.506

Note that part of the gain comes from bigger transforms
(8x8 instead of 4x4) and part comes from replacing the DCT
wth the ADST.

Change-Id: I92ca6bbfce11b4165d612b81d9adfad4d010c775