Adaptive multiaxis step smoothing

- Stepper bugs fixed
- Support MIXING_EXTRUDER with Linear Advance
- Miscellaneous cleanup

Marlin/src/module/planner.cpp

@@ -679,9 +679,9 @@ void Planner::init() {
       return r11 | (uint16_t(r12) << 8) | (uint32_t(r13) << 16);
     }
   #else
-    // All the other 32 CPUs can easily perform the inverse using hardware division,
+    // All other 32-bit MPUs can easily do inverse using hardware division,
     // so we don't need to reduce precision or to use assembly language at all.
-    // This routine, for all the other archs, returns 0x100000000 / d ~= 0xFFFFFFFF / d
+    // This routine, for all other archs, returns 0x100000000 / d ~= 0xFFFFFFFF / d
     static FORCE_INLINE uint32_t get_period_inverse(const uint32_t d) { return 0xFFFFFFFF / d; }
   #endif
 #endif
@@ -1646,10 +1646,16 @@ bool Planner::_populate_block(block_t * const block, bool split_move,
   // Bail if this is a zero-length block
   if (block->step_event_count < MIN_STEPS_PER_SEGMENT) return false;
 
-  // For a mixing extruder, get a magnified step_event_count for each
+  // For a mixing extruder, get a magnified esteps for each
   #if ENABLED(MIXING_EXTRUDER)
     for (uint8_t i = 0; i < MIXING_STEPPERS; i++)
-      block->mix_event_count[i] = mixing_factor[i] * block->step_event_count;
+      block->mix_steps[i] = mixing_factor[i] * (
+        #if ENABLED(LIN_ADVANCE)
+          esteps
+        #else
+          block->step_event_count
+        #endif
+      );
   #endif
 
   #if FAN_COUNT > 0