GPv3: Fix Remove Material Slot and Unused Slots

There was two problems:

* If you remove the slot, the material was not reasigned
and the attribute could get a wrong index.
* The Unused Materials operator was executed, but
it was doing nothing.

@@ -1393,0 +1401,4 @@
+
+    greasepencil::Drawing &drawing = reinterpret_cast<GreasePencilDrawing *>(base)->wrap();
+    MutableAttributeAccessor attributes = drawing.strokes_for_write().attributes_for_write();
+    SpanAttributeWriter<int> material_indices = attributes.lookup_or_add_for_write_span<int>(

@@ -1393,0 +1430,4 @@
+    greasepencil::Drawing &drawing = reinterpret_cast<GreasePencilDrawing *>(base)->wrap();
+    AttributeAccessor attributes = drawing.strokes().attributes();
+    const VArraySpan<int> material_indices = *attributes.lookup_or_default<int>(
+        "material_index", ATTR_DOMAIN_CURVE, -1);

@@ -1393,0 +1411,4 @@
+    for (const int i : indices_span.index_range()) {
+      if ((indices_span[i] > index) || (indices_span[i] > totcol - 1)) {
+        indices_span[i]--;
+        indices_span[i] = std::max(indices_span[i], 0);

@@ -1390,6 +1390,55 @@ void BKE_grease_pencil_material_remap(GreasePencil *grease_pencil, const uint *r
   }
 }
 
+void BKE_grease_pencil_material_index_reassign(GreasePencil *grease_pencil, int totcol, int index)

@@ -1393,0 +1409,4 @@
+
+    MutableVArraySpan<int> indices_span(material_indices.varray);
+    for (const int i : indices_span.index_range()) {
+      if ((indices_span[i] > index) || (indices_span[i] > totcol - 1)) {

@@ -1370,2 +1372,4 @@
     BKE_gpencil_material_index_reassign((bGPdata *)ob->data, ob->totcol, actcol - 1);
   }
+  /* check indices from greease pencil. */
+  else if (ob->type == OB_GREASE_PENCIL) {

@@ -464,2 +464,4 @@
       BKE_curve_material_index_remove((Curve *)id, index);
       break;
+    case ID_GP:
+      BKE_grease_pencil_material_index_remove((GreasePencil *)id, index);