New content type to improve grain retention.

For new VP9 only content type adjust  the rate distortion and ARF
filter based on the relative spatial variance of the source and
reconstruction.

In regards to the RD loop the method favors modes where the
reconstruction variance is similar to the source variance. However it
is currently only applied to regions where the source variance is quite
low.

For very low variance blocks it applies a further bias against intra
coding and large prediction block sizes (the later in particular limit
the usefulness of the loop filter).

The final part of this change is to lower the strength of the ARF
filter for blocks where the source has very low spatial variance, to
encourage some low amplitude texture or noise to pass through
the filter.

This change improves the retention of film grain and fine noise /
texture in spatially flat regions, but as expected causes a significant
drop in PSNR on many clips. This is to be expected because similar
but misaligned noise or texture will give a lower PSNR than a flat
noise free reconstruction. However, it is worth noting that most clips
show a strong gain in FAST SSIM.

The features are enabled on the vpxenc command line by setting
--tune-content=film.

VPX_ENCODER_ABI_VERSION bumped for this change and cvbr.

Change-Id: I26a4e4edfa3dc5cacead82fa701fe7a9118ccd0a

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