Add `remove_frame` function to layers

@@ -288,2 +300,4 @@
   GreasePencilFrame *add_frame_internal(int frame_number, int drawing_index);
   int frame_index_at(int frame_number) const;
+  /**
+   * Removes null frames starting from \a begin until \a end or a non-null frame is reached.

@@ -603,1 +603,4 @@
 
+const int *Layer::remove_leading_null_frames_in_range(const int *begin, const int *end)
+{
+  auto next_it = begin;

@@ -665,0 +694,4 @@
+    /* If the previous frame is not an implicit hold (e.g. it has a fixed duration) and it's not a
+     * null frame, we cannot just delete the frame. We need to replace it with a null frame. */
+    if (!prev_frame.is_implicit_hold() && !prev_frame.is_null()) {
+      this->frames_for_write().lookup(start_frame_number) = GreasePencilFrame::null();

@@ -290,0 +304,4 @@
+   * \param begin, end: Iterators into the `sorted_keys` span.
+   * \returns an iterator to the element after the last null-frame that was removed.
+   */
+  const int *remove_leading_null_frames_in_range(const int *begin, const int *end);