Generalized deblocking filter for AVM

The AV1 deblocking filter did not sufficiently attenuate visual quality artifacts when used in the AOMedia video model (AVM), especially at lower bitrates. A generalized deblocking filter described in this paper uses one equation for any filter length. The filter brings PSNR-YUV BD-rate $-\mathbf{0. 1 7 \%},\ -0.90\%,-1.07\%$, and $-0.92\%$ on All Intra, Random Access, LowDelay, and Adaptive Streaming configurations in the AOMedia common test conditions. Improvement of visual quality is observed on a number of sequences, while the computational complexity is close to that of the AVM deblocking.