new layer model

kiln-core/src/object_def.rs

@@ -34,10 +34,6 @@ pub struct ObjectDef {
     pub x: usize,
     /// Row of this object on the board (0-indexed).
     pub y: usize,
-    /// Index of the layer this object belongs to (0 = bottom). Determines its
-    /// draw order: objects on higher layers render above lower-layer terrain and
-    /// objects. Set at load from the layer the object's palette char appeared in.
-    pub z: usize,
     /// Visual representation of this object. Owned by the object (not derived
     /// from the grid cell), so scripts can change tile, fg, and bg at runtime.
     pub glyph: Glyph,
@@ -101,7 +97,6 @@ impl ObjectDef {
             id: 0,
             x,
             y,
-            z: 0,
             glyph: Self::default_glyph(),
             solid: true,
             opaque: true,