Fix: Stepping issue with Butterworth filter

The same issue for Smooth (Gaussian) which was reported in #109799 also affects the Butterworth filter.
When keys were not exactly on the frame, but offset even by minimal values the filter would introduce stepping.
Fix it by using `round()` instead of `(int)` when calculating the index to the filtered values array

Pull Request: https://projects.blender.org/blender/blender/pulls/110060

source/blender/editors/animation/keyframes_general.cc

@@ -557,7 +557,9 @@ void butterworth_smooth_fcurve_segment(FCurve *fcu,
     }
 
     const float x_delta = fcu->bezt[i].vec[1][0] - left_bezt.vec[1][0] + filter_order;
-    const int filter_index = (int)(x_delta * sample_rate);
+    /* Using round() instead of casting to int. Casting would introduce a stepping issue when the
+     * x-value is just below a full frame. */
+    const int filter_index = round(x_delta * sample_rate);
     const float blend_value = butterworth_calculate_blend_value(samples,
                                                                 filtered_values,
                                                                 samples_start_index,