Fix a bug in calculating the compound ref frame cost
Fix a bug in calculating the compound ref frame cost

The previous ext-refs experiment did not consider the cost of the 2nd
reference frame on mode decision in the compound mode. With the fix,
using Overall PSNR, compared to the previous ext-refs RD performance
before the bug fix, all against the baseline, the improvements are:

"ext-refs" before fix: lowres -5.665%  midres: -4.833%
"ext-refs" after fix:  lowres -5.776%  midres: -5.000%
Improvement by the fix: lowres -0.111%  midres: -0.167%

Change-Id: I2eceedf2d4046b169514e049fd01baaf0bbb50c6

smarter/aom,luctrudeau/aom,GrokImageCompression/aom,mbebenita/aom,smarter/aom,luctrudeau/aom,mbebenita/aom,mbebenita/aom,mbebenita/aom,GrokImageCompression/aom,mbebenita/aom,mbebenita/aom,smarter/aom,luctrudeau/aom,mbebenita/aom,smarter/aom,GrokImageCompression/aom,GrokImageCompression/aom,luctrudeau/aom,GrokImageCompression/aom,mbebenita/aom,GrokImageCompression/aom,mbebenita/aom,smarter/aom,smarter/aom,luctrudeau/aom,luctrudeau/aom