Minor optimization of FILAMENT_WIDTH_SENSOR

Marlin/temperature.cpp

     if (filament_sensor) {
       meas_shift_index = filwidth_delay_index[0] - meas_delay_cm;
       if (meas_shift_index < 0) meas_shift_index += MAX_MEASUREMENT_DELAY + 1;  //loop around buffer if needed
+      meas_shift_index = constrain(meas_shift_index, 0, MAX_MEASUREMENT_DELAY);
 
       // Get the delayed info and add 100 to reconstitute to a percent of
       // the nominal filament diameter then square it to get an area
-      meas_shift_index = constrain(meas_shift_index, 0, MAX_MEASUREMENT_DELAY);
-      float vm = pow((measurement_delay[meas_shift_index] + 100.0) * 0.01, 2);
-      NOLESS(vm, 0.01);
-      volumetric_multiplier[FILAMENT_SENSOR_EXTRUDER_NUM] = vm;
+      const float vmroot = measurement_delay[meas_shift_index] * 0.01 + 1.0;
+      volumetric_multiplier[FILAMENT_SENSOR_EXTRUDER_NUM] = vmroot <= 0.1 ? 0.01 : sq(vmroot);
     }
   #endif // FILAMENT_WIDTH_SENSOR