Revert incorrect impl Arbitrary for Zoom.

I made this mistake in cfe2e93.
We need the custom implementation to avoid generating out-of-range
offsets.

all-is-cubes/src/block/modifier/zoom.rs

@@ -13,7 +13,6 @@ use crate::universe;
 /// Design note: This is a struct separate from [`Modifier`] so that it can have a
 /// constructor accepting only valid bounds.
 #[derive(Clone, Debug, Eq, Hash, PartialEq)]
-#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
 pub struct Zoom {
     /// Scale factor to zoom in by.
     scale: Resolution,
@@ -155,6 +154,30 @@ impl universe::VisitHandles for Zoom {
     }
 }
 
+#[cfg(feature = "arbitrary")]
+impl<'a> arbitrary::Arbitrary<'a> for Zoom {
+    fn arbitrary(u: &mut arbitrary::Unstructured<'a>) -> arbitrary::Result<Self> {
+        let scale = u.arbitrary()?;
+        let max_offset = GridCoordinate::from(scale) - 1;
+        Ok(Self::new(
+            scale,
+            GridPoint::new(
+                u.int_in_range(0..=max_offset)?,
+                u.int_in_range(0..=max_offset)?,
+                u.int_in_range(0..=max_offset)?,
+            ),
+        ))
+    }
+
+    fn size_hint(depth: usize) -> (usize, Option<usize>) {
+        use arbitrary::{size_hint::and_all, Arbitrary};
+        and_all(&[
+            <Resolution as Arbitrary>::size_hint(depth),
+            <[GridCoordinate; 3] as Arbitrary>::size_hint(depth),
+        ])
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;