Region paint order is random unless every region sets z #14

{ type: "palette", name: "p", colors: { _: "transparent", a: "#ff0000", b: "#0000ff", c: "#00ff00", d: "#ffff00" } }
{ type: "sprite", name: "order", size: [4, 4], palette: "p", regions: {
    a: { rect: [0, 0, 4, 4] },
    b: { rect: [1, 1, 3, 3] },
    c: { rect: [2, 2, 2, 2] },
    d: { points: [[3, 3]] },
} }

The docs (format/regions.md, "Z-Order") say the default is definition order.
Sprite.regions is a HashMap, though, and render_structured stable-sorts
by z, so equal-z regions paint in hash order. That order changes from process
to process: rendering the repro 6 times gave 3 different PNGs. Workaround:
every region in skull.pxl has an explicit, unique z. The fix is probably an
IndexMap for regions, which would also make fill/except dependency
order match the source.

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

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