Initial camera camera repositioning

ray-tracing-core/src/math/pos3.rs

@@ -71,6 +71,18 @@ impl Add<Dir3> for Pos3 {
     }
 }
 
+impl Sub<Dir3> for Pos3 {
+    type Output = Pos3;
+
+    fn sub(self, rhs: Dir3) -> Self::Output {
+        Pos3 {
+            x: self.x - rhs.x,
+            y: self.y - rhs.y,
+            z: self.z - rhs.z,
+        }
+    }
+}
+
 impl Index<usize> for Pos3 {
     type Output = Float;