Adds a special combination mode specific to intra prediciton
mode D45.

Current results with the compound inter/intra experiment:
derf: 0.2%
yt: 0.55%
std-hd: 0.75%
hd: 0.74%

Change-Id: I8976bdf3b9b0b66ab8c5c628bbc62c14fc72ca86