Adjustments to key frame sizing.

Adjustments to key frame sizing.

Adjustments take heavier account of the frame near a kf
in deciding boost and limit the total number that can contribute.
Also adjusted the minq calculations such that in most cases we
generate a smaller key frame.
Modified the code that accounts for how static the sequence is and
added some adjustment based on image size. This is still very
crude but smaller images tend to behave better with a larger
delta between KF Q and other frames than larger image formats.
Changes give sizable gains in overall PSNR  on all the test sets but the
biggest gains (~3%) were on the std-hd set.
The gains were smaller for SSIM but still significant.
Average PSNR results are mixed because this metric can very easily
be altered by having a very good / lossless coding of one or two frames.
Some of the YT and YT-HD clips in particular have blank lead ins and
allowing lossless coding of these appears to make a big difference to
average PSNR but it reality does not help much at all.

Change-Id: I6bfe485a1d330b47c783832f1717c95c535464ec