Guard against planning/numerical errors in short segments

Turns out for high-res curved models the numerical error and the
SLOWDOWN handling in the planner can cause enough variance in the
calculated pressure to trigger LA to continuosly, making matters worse.

Clamp LA again, but only during extrusion, so that the runaway error is
limited by the current segment length.

Firmware/stepper.cpp

@@ -809,8 +809,11 @@ FORCE_INLINE void isr() {
         acceleration_time += timer;
 #ifdef LIN_ADVANCE
         if (current_block->use_advance_lead) {
-            if (step_events_completed.wide <= (unsigned long int)step_loops)
+            if (step_events_completed.wide <= (unsigned long int)step_loops) {
                 la_state = ADV_INIT | ADV_ACC_VARY;
+                if (e_extruding && current_adv_steps > target_adv_steps)
+                    target_adv_steps = current_adv_steps;
+            }
         }
 #endif
       }
@@ -832,6 +835,8 @@ FORCE_INLINE void isr() {
             if (step_events_completed.wide <= (unsigned long int)current_block->decelerate_after + step_loops) {
                 target_adv_steps = current_block->final_adv_steps;
                 la_state = ADV_INIT | ADV_ACC_VARY;
+                if (e_extruding && current_adv_steps < target_adv_steps)
+                    target_adv_steps = current_adv_steps;
             }
         }
 #endif
@@ -849,6 +854,8 @@ FORCE_INLINE void isr() {
               // acceleration or deceleration can be skipped or joined with the previous block.
               // If LA was not previously active, re-check the pressure level
               la_state = ADV_INIT;
+              if (e_extruding)
+                  target_adv_steps = current_adv_steps;
           }
 #endif
         }