Fix kernel_radius function for SSIM scaler

The SSIM class does not have a `ref` property, so it throws
a runtime error. The change uses the `scaler` property instead,
which exists.

vsscale/scale.py

@@ -142,7 +142,7 @@ def _linear_scale(
 
     @inject_self.property
     def kernel_radius(self) -> int:
-        return self.ref.kernel_radius
+        return self.scaler.kernel_radius
 
 
 @dataclass