Fix #118509: Refactored the GPv3 drawing array compression function.

The `remove_drawings_with_no_users` did not work properly when removing
more than one drawing at a time. It created incorrect drawing indices
that were larger than the drawings array, causing crashes down the line.

The new implementation should be both cleaner and more efficient,
avoiding a loop over all frames for every drawing removed.

@@ -2310,0 +2255,4 @@
+  };
+
+  /* Index map to remap drawing indices in frame data. */
+  Array<int> drawing_index_map(drawings.size(), -1);

@@ -2310,0 +2259,4 @@
+
+  int first_unused_drawing = -1;
+  int last_used_drawing = drawings.size();
+  while (true) {

@@ -2310,0 +2301,4 @@
+  }
+
+  /* Remap drawing indices in frame data. */
+  for (Layer *layer : this->layers_for_write()) {