Force even arf group length where possible.
Force even arf group length where possible.

This patch tweaks the calculation of the active maximum GF interval
and also the break out clause for the GF interval loop. The changes
force the maximum and where possible the  break out value to be odd
which in turn will result in an even length ARF group if ARF coding is
selected (vs GF only coding).

The primary aim was to improve coding with multi layer arf groups.
For the single layer case there are small net gains in 3 out of 4 sets
(low,md, hd) and a small net drop for the NF2K set.

For multi-layer the gains (opsnr, ssim, psnr-hvs : -ve = better) were:-

Low res: -0.109, -0.038, -0.036
Mid res: -0.204, -0.171, -0.242
Hd res: -0.330, -0.471, -0.496
NF 2k: -0.165, -0.149, -0.157

Change-Id: I245f8561f5d1bd34312a0133c670c2154a0da23f

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