Make M421 more versatile

Marlin/src/gcode/bedlevel/abl/M421.cpp

 #endif
 
 /**
- * M421: Set a single Mesh Bed Leveling Z coordinate
+ * M421: Set one or more Mesh Bed Leveling Z coordinates
  *
  * Usage:
  *   M421 I<xindex> J<yindex> Z<linear>
  *   M421 I<xindex> J<yindex> Q<offset>
+ *
+ *  - If I is omitted, set the entire row
+ *  - If J is omitted, set the entire column
+ *  - If both I and J are omitted, set all
  */
 void GcodeSuite::M421() {
   int8_t ix = parser.intval('I', -1), iy = parser.intval('J', -1);
-  const bool hasI = ix >= 0,
-             hasJ = iy >= 0,
-             hasZ = parser.seen('Z'),
-             hasQ = !hasZ && parser.seen('Q');
+  const bool hasZ = parser.seenval('Z'),
+             hasQ = !hasZ && parser.seenval('Q');
 
-  if (!hasI || !hasJ || !(hasZ || hasQ))
-    SERIAL_ERROR_MSG(STR_ERR_M421_PARAMETERS);
-  else if (!WITHIN(ix, 0, GRID_MAX_POINTS_X - 1) || !WITHIN(iy, 0, GRID_MAX_POINTS_Y - 1))
-    SERIAL_ERROR_MSG(STR_ERR_MESH_XY);
-  else {
-    z_values[ix][iy] = parser.value_linear_units() + (hasQ ? z_values[ix][iy] : 0);
-    TERN_(ABL_BILINEAR_SUBDIVISION, bed_level_virt_interpolate());
-    TERN_(EXTENSIBLE_UI, ExtUI::onMeshUpdate(ix, iy, z_values[ix][iy]));
+  if (hasZ || hasQ) {
+    if (WITHIN(ix, -1, GRID_MAX_POINTS_X - 1) && WITHIN(iy, -1, GRID_MAX_POINTS_Y - 1)) {
+      const float zval = parser.value_linear_units();
+      uint8_t sx = ix >= 0 ? ix : 0, ex = ix >= 0 ? ix : GRID_MAX_POINTS_X - 1,
+              sy = iy >= 0 ? iy : 0, ey = iy >= 0 ? iy : GRID_MAX_POINTS_Y - 1;
+      LOOP_S_LE_N(x, sx, ex) {
+        LOOP_S_LE_N(y, sy, ey) {
+          z_values[x][y] = zval + (hasQ ? z_values[x][y] : 0);
+          TERN_(EXTENSIBLE_UI, ExtUI::onMeshUpdate(x, y, z_values[x][y]));
+        }
+      }
+      TERN_(ABL_BILINEAR_SUBDIVISION, bed_level_virt_interpolate());
+    }
+    else
+      SERIAL_ERROR_MSG(STR_ERR_MESH_XY);
   }
+  else
+    SERIAL_ERROR_MSG(STR_ERR_M421_PARAMETERS);
 }
 
 #endif // AUTO_BED_LEVELING_BILINEAR