GPv3: simplify adding multiple layers with empty drawings

This is often required when generating grease pencil procedurally, e.g. in #124279.

source/blender/blenkernel/intern/grease_pencil.cc

@@ -2403,6 +2403,23 @@ bool GreasePencil::remove_frames(blender::bke::greasepencil::Layer &layer,
   return false;
 }
 
+void GreasePencil::add_layers_with_empty_drawings_for_eval(const int num)
+{
+  using namespace blender;
+  using namespace blender::bke::greasepencil;
+  const int old_drawings_num = this->drawing_array_num;
+  this->add_empty_drawings(num);
+  for (const int i : IndexRange(num)) {
+    const int drawing_i = old_drawings_num + i;
+    Drawing &drawing = reinterpret_cast<GreasePencilDrawing *>(this->drawing(drawing_i))->wrap();
+    Layer &layer = this->add_layer(std::to_string(i));
+    GreasePencilFrame *frame = layer.add_frame(this->runtime->eval_frame);
+    BLI_assert(frame);
+    frame->drawing_index = drawing_i;
+    drawing.add_user();
+  }
+}
+
 void GreasePencil::remove_drawings_with_no_users()
 {
   using namespace blender;

source/blender/makesdna/DNA_grease_pencil_types.h

@@ -606,6 +606,12 @@ typedef struct GreasePencil {
    */
   bool remove_frames(blender::bke::greasepencil::Layer &layer, blender::Span<int> frame_numbers);
 
+  /**
+   * Adds multiple layers each with its own empty drawing. This can be more efficient than adding
+   * every layer and drawing one by one.
+   */
+  void add_layers_with_empty_drawings_for_eval(int num);
+
   /**
    * Low-level resizing of drawings array. Only allocates new entries in the array, no drawings are
    * created in case of size increase. In case of size decrease, the removed drawings are deleted.