Cycles: Fix command line render overshooting time limit

The calculation based on preserving device occupancy was conflicting
with the fact that time limit needs to render less samples at the last
round of render work.

For example, rendering BMW27 for 30sec on i9-11900k was actually
rendering for almost a minute. Now the render time limit is respected
much more close.

Differential Revision: https://developer.blender.org/D13269

intern/cycles/integrator/render_scheduler.cpp

@@ -827,6 +827,26 @@ int RenderScheduler::get_num_samples_to_path_trace() const
       num_samples_to_occupy = lround(state_.occupancy_num_samples * 0.7f / state_.occupancy);
     }
 
+    /* When time limit is used clamp the calculated number of samples to keep occupancy.
+     * This is because time limit causes the last render iteration to happen with less number of
+     * samples, which conflicts with the occupancy (lower number of samples causes lower
+     * occupancy, also the calculation is based on number of previously rendered samples).
+     *
+     * When time limit is not used the number of samples per render iteration is either increasing
+     * or stays the same, so there is no need to clamp number of samples calculated for occupancy.
+     */
+    if (time_limit_ && state_.start_render_time) {
+      const double remaining_render_time = max(
+          0.0, time_limit_ - (time_dt() - state_.start_render_time));
+      const double time_per_sample_average = path_trace_time_.get_average();
+      const double predicted_render_time = num_samples_to_occupy * time_per_sample_average;
+
+      if (predicted_render_time > remaining_render_time) {
+        num_samples_to_occupy = lround(num_samples_to_occupy *
+                                       (remaining_render_time / predicted_render_time));
+      }
+    }
+
     num_samples_to_render = max(num_samples_to_render,
                                 min(num_samples_to_occupy, max_num_samples_to_render));
   }