bvh: don't require T to implement Default

src/bvh.rs

@@ -19,11 +19,18 @@ impl<R,T> Default for RecursiveContent<R,T>{
 		Self::Branch(Vec::new())
 	}
 }
-#[derive(Default)]
 pub struct BvhNode<T>{
 	content:RecursiveContent<BvhNode<T>,T>,
 	aabb:Aabb,
 }
+impl<T> Default for BvhNode<T>{
+	fn default()->Self{
+		Self{
+			content:Default::default(),
+			aabb:Aabb::default(),
+		}
+	}
+}
 pub struct BvhWeightNode<W,T>{
 	content:RecursiveContent<BvhWeightNode<W,T>,T>,
 	weight:W,