all: add support for hy3 like tiling

src/gfx_api.rs

@@ -306,6 +306,8 @@ pub struct RoundedFillRect {
     pub border_width: f32,
     /// Output scale for antialiasing.
     pub scale: f32,
+    /// Sort order hint within the RoundedFill bucket (lower renders first).
+    pub z_order: u32,
 }
 
 impl RoundedFillRect {