Grid: make buf and blur private

src/grid.rs

@@ -46,9 +46,9 @@ pub struct Grid {
     pub data: Vec<f32>,
 
     // Scratch space for the blur operation.
-    // pub buf: Vec<f32>,
-    pub buf: Buf,
-    pub blur: Blur,
+    buf: Buf,
+
+    blur: Blur,
     pub agents: Vec<Agent>,
 }