Derived sprites (source:) still require palette, contrary to docs

The docs example { type: "sprite", name: "hero_outlined", source: "hero", transform: [...] } fails with missing field 'palette'. In skull.pxl,
the composition after that line then wasn't found either. Workaround: not needed after gap 4, but any
source: sprite has to repeat palette.

Found while drawing examples/skull (#13). The repro is checked against main.