The commit improves the 32x32 forward dct implementation:
1. change to use same constants and rounding as other forward dcts
2. select rounding to specifically minimize the roundtrip error, which
improved average 19/block to .77/block using 100000 random input.

Test showed a small but consistent gain on all test sets, about .15%

Change-Id: If0afd6a71880a522f60c1c234be0462092c2eb53