Add (currently non-functional) example of using custom data in a filter.

examples/builder_data.rs

@@ -0,0 +1,35 @@
+use mapgen::{
+    filter::{
+        AreaStartingPosition, CellularAutomata, CullUnreachable, NoiseGenerator, XStart, YStart,
+    },
+    BuilderData, MapBuilder, MapFilter,
+};
+
+#[derive(Clone, Default)]
+struct MyData {
+    value: usize,
+}
+
+impl BuilderData for MyData {}
+
+struct IncrementData;
+
+impl<D: BuilderData> MapFilter<D> for IncrementData {
+    fn modify_map(&self, rng: &mut rand::prelude::StdRng, map: &mapgen::Map<D>) -> mapgen::Map<D> {
+        let map = map.clone();
+        map.data.value += 1;
+        map
+    }
+}
+
+fn main() {
+    let map = MapBuilder::<MyData>::new(20, 20)
+        .with(NoiseGenerator::uniform())
+        .with(CellularAutomata::new())
+        .with(AreaStartingPosition::new(XStart::CENTER, YStart::CENTER))
+        .with(CullUnreachable::new())
+        .with(Box::new(IncrementData))
+        .build();
+
+    println!("{:}", &map);
+}