Alpha encoded with AOM_CICP_MC_IDENTITY

[y-guyon]

For color:

libavif/src/codec_aom.c

Line 1071 in 632b7ea

aomImage.mc = (aom_matrix_coefficients_t)image->matrixCoefficients;

But for alpha, mc is left to its default value (0 because of memset() above), which means AOM_CICP_MC_IDENTITY, which is forbidden:

subsampling_x	subsampling_y	mono_chrome	Description
0	0	0	YUV 4:4:4
1	0	0	YUV 4:2:2
1	1	0	YUV 4:2:0
1	1	1	Monochrome 4:0:0

If matrix_coefficients is equal to MC_IDENTITY, it is a requirement of bitstream conformance that subsampling_x is equal to 0 and subsampling_y is equal to 0.