Raise Z while preheating when auto/[un]loading the filament

Preheating already raised Z to avoid scorching the PEI sheet, as does
filament loading/unloading to allow for excess material to be removed.

However, when loading/autoloading/unloading via the LCD with a cold
nozzle the preheating menu is performed before the carriage is raised,
leaving the carriage close to the sheet while heating the nozzle.

Pre-raise the carriage already while waiting, so that the subsequent
move is automagically skipped.

Set bFilamentWaitingFlag only once to perform both the LCD
initialization and raising to the appropriate height.

Should fix prusa3d#2761

Firmware/ultralcd.cpp

@@ -2412,42 +2412,53 @@ void mFilamentItem(uint16_t nTemp, uint16_t nTempBed)
         case FilamentAction::None:
         case FilamentAction::Preheat:
         case FilamentAction::Lay1Cal:
-
+            // handled earlier
             break;
         }
         if (bFilamentWaitingFlag) Sound_MakeSound(e_SOUND_TYPE_StandardPrompt);
         bFilamentWaitingFlag = false;
     }
     else
     {
-        bFilamentWaitingFlag = true;
         lcd_set_cursor(0, 0);
         lcdui_print_temp(LCD_STR_THERMOMETER[0], (int) degHotend(0), (int) degTargetHotend(0));
-        lcd_set_cursor(0, 1);
-        switch (eFilamentAction)
+
+        if (!bFilamentWaitingFlag)
         {
-        case FilamentAction::Load:
-        case FilamentAction::AutoLoad:
-        case FilamentAction::MmuLoad:
-            lcd_puts_P(_i("Preheating to load")); ////MSG_ c=20
-            break;
-        case FilamentAction::UnLoad:
-        case FilamentAction::MmuUnLoad:
-            lcd_puts_P(_i("Preheating to unload")); ////MSG_ c=20
-            break;
-        case FilamentAction::MmuEject:
-            lcd_puts_P(_i("Preheating to eject")); ////MSG_ c=20
-            break;
-        case FilamentAction::MmuCut:
-            lcd_puts_P(_i("Preheating to cut")); ////MSG_ c=20
-            break;
-        case FilamentAction::None:
-        case FilamentAction::Preheat:
-        case FilamentAction::Lay1Cal:
-            break;
+            // First run after the filament preheat selection:
+            // setup the fixed LCD parts and raise Z as we wait
+            bFilamentWaitingFlag = true;
+
+            lcd_set_cursor(0, 1);
+            switch (eFilamentAction)
+            {
+            case FilamentAction::Load:
+            case FilamentAction::AutoLoad:
+            case FilamentAction::MmuLoad:
+                lcd_puts_P(_i("Preheating to load")); ////MSG_ c=20
+                raise_z_above(MIN_Z_FOR_LOAD);
+                break;
+            case FilamentAction::UnLoad:
+            case FilamentAction::MmuUnLoad:
+                lcd_puts_P(_i("Preheating to unload")); ////MSG_ c=20
+                raise_z_above(MIN_Z_FOR_UNLOAD);
+                break;
+            case FilamentAction::MmuEject:
+                lcd_puts_P(_i("Preheating to eject")); ////MSG_ c=20
+                break;
+            case FilamentAction::MmuCut:
+                lcd_puts_P(_i("Preheating to cut")); ////MSG_ c=20
+                break;
+            case FilamentAction::None:
+            case FilamentAction::Preheat:
+            case FilamentAction::Lay1Cal:
+                // handled earlier
+                break;
+            }
+            lcd_set_cursor(0, 3);
+            lcd_puts_P(_i(">Cancel"));                   ////MSG_ c=20 r=1
         }
-        lcd_set_cursor(0, 3);
-        lcd_puts_P(_i(">Cancel"));                   ////MSG_ c=20 r=1
+
         if (lcd_clicked())
         {
             bFilamentWaitingFlag = false;