Add logic to compute channel grids for modular.

[veluca93]

Also compute the relevant transforms "decomposed" to operate on grid chunks, and the dependencies in decoding (i.e. which inputs are needed for which transform).

[jonsneyers]

LGTM. It would be useful to create a set of test bitstreams (in addition to the conformance set), maybe using jxl_from_tree (though some extra stuff will need to be exposed such as delta palette predictor choice/nb_deltas, and progressive scans info), to test exotic combinations of modular transforms. The libjxl encoder doesn't produce weird combinations like delta palette plus squeeze...

[veluca93]

LGTM. It would be useful to create a set of test bitstreams (in addition to the conformance set), maybe using jxl_from_tree (though some extra stuff will need to be exposed such as delta palette predictor choice/nb_deltas, and progressive scans info), to test exotic combinations of modular transforms. The libjxl encoder doesn't produce weird combinations like delta palette plus squeeze...

Yup, we definitely should check that at some point or another...