This commit tries to address an issue related to the oddity shown on
HD _mobcal clip, where some rather ugly blocks shown in the second
frame at low-mid bit rates if the third frame is not made a key frame
by he encoder. The fixes include: 1)  made calls to sad_16x16 to be
consistent with function prototype. 2) remove the error bias to intra
and golden in mbgraph search. 3) changed the error accumulation on
inter_segment encoding to avoid potential out-of-range. 1) has no
effect on encoding results.

Encoding test show that the overall effect of the commit helps about
.2%(HD) to .3%(cif)

Change-Id: I930975a2d0c06252f01c39e0a02351529774e30b