refactors mapgen into chained builders

src/map_builders/room_based_starting_position.rs

@@ -0,0 +1,26 @@
+use super::{BuilderMap, MetaMapBuilder, Position};
+use rltk::RandomNumberGenerator;
+
+pub struct RoomBasedStartingPosition {}
+
+impl MetaMapBuilder for RoomBasedStartingPosition {
+    fn build_map(&mut self, rng: &mut rltk::RandomNumberGenerator, build_data: &mut BuilderMap) {
+        self.build(rng, build_data);
+    }
+}
+
+impl RoomBasedStartingPosition {
+    #[allow(dead_code)]
+    pub fn new() -> Box<RoomBasedStartingPosition> {
+        Box::new(RoomBasedStartingPosition {})
+    }
+
+    fn build(&mut self, _rng: &mut RandomNumberGenerator, build_data: &mut BuilderMap) {
+        if let Some(rooms) = &build_data.rooms {
+            let start_pos = rooms[0].centre();
+            build_data.starting_position = Some(Position { x: start_pos.0, y: start_pos.1 });
+        } else {
+            panic!("RoomBasedStartingPosition only works after rooms have been created");
+        }
+    }
+}