Adapt GOP size threshold to the allowed layer depth

Adapt GOP size threshold to the allowed layer depth

Increase the total prediction error budget linearly with the
allowed ARF layer depth. This in general improves the compression
performance, but does hit corner cases on a few clips at very
low bit-rate range (corresponding to 26 - 28 dB range). To mitigate
such problem, we temporarily work around this problem by limiting
the first GOP size to be ~8 so as to not drain up the bit resource.

The overall compression performance improvements over the current
multi-layer ARF system in speed 0 are:

           overall PSNR      avg PSNR        SSIM
lowres     -0.47%            -0.13%         -1.51%
midres     -1.30%            -1.16%         -2.80%
hdres      -0.91%            -0.84%         -2.15%

Change-Id: Ia4880ab63e98e15a9db99aea6eabfd3d1da9270d