swept_blocks_match_the_pixel_search skips the cases where the colour mode reaches extrude only
through a channel count and ink setting an earlier mode already covered. It and
damaged_layer_data_opens_flattened run a quarter of their cases per run, a different quarter each
UTC day (photocraft-testkit's Sweep). PHOTOCRAFT_FULL_SWEEP=1 runs every case, and
PHOTOCRAFT_SWEEP_PART=<n> replays the part a failing run printed.