Fix KeyFrame generation rate increasing over time

[sdencanted]

In frontend/fullsystem.cc Line 133, the members .a and .b of fh->aff_g2l() are both always 0. This causes refToFh in line 166 to increase gradually over time, which in turn causes the variable b in line 168 to be larger than 1 more frequently. This causes the keyframe generation and thus latency to increase over time significantly when running on long videos.

I was able to keep the keyframe generation rate stable by changing fh->aff_g2l() to fh->frame->aff_g2l, whose members .a and .b are non-zero.