Just before a forced key frame we often get a foreshortened
arf/gf group. In such a case, we do not want to update
rc->last_boosted_qindex, which is used to define the Q range
for the forced key frame itself.

This gives a small average metrics gain for the YT and YT-HD sets
(eg. YT SSIM +0.141%).

Change-Id: Ie06698bc4f249e87183b8f8fb27ff8f3fde216d9