add an RGB (BT709/sRGB/Identity) sample
Created by converting ToS pngs to yuv444p y4m using libavfilter/libswscale, then to AV1 with aomenc --transfer-characteristics=srgb --matrix-coefficients=identity --color-primaries=bt709
md5 output of aomdec --md5 --rawvideo rgb.ivf
matches dav1d -i rgb.ivf --muxer md5 -o -