Add min gf group length for groups containing a fwd keyframe
Add min gf group length for groups containing a fwd keyframe

Results on 150 frames of lowres:

KF Interval           BDRATE
5                    -19.944%
10                   -19.904%
30                   -10.959%
60                   -6.539%

Change-Id: Iec5ed0d2c0b812906dac68ab852280895ec9b131

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom