skPaper (shader module)

The paper field, for blocks that list paper under reads and declare sk_paper (a texture, default black) after the shared header. It says how the sheet under each pixel takes paint and ink: sk_thirst(uv) (-1 hard-sized, 0 the standard sheet blocks are tuned on, +1 thirsty), sk_sheen(uv), sk_fibre(uv) and sk_dark_ground(uv) (1 where the picture is made of light marks on a dark sheet). A Ground block publishes it with a field.paper shader in its block.yaml; where none does every value is 0, so a block reading it draws exactly as before. The field is only made when some live block reads it, so it costs nothing otherwise. paperBlocks' papers publish it; watercolorBlocks' bleed and edge darkening and every inkBlocks line and fill read it.