Grease Pencil provides custom vertex group attributes as VArrays that
are a view on the `MDeformVert` buffer. These attributes are not spans,
which the curve conversion code was expecting. Non-span VArrays must be
materialized first.

@@ -381,0 +383,4 @@
+
+    buffer.reinitialize(curves.evaluated_points_num() * src.type().size());
+    GMutableSpan eval{src.type(), buffer.data(), curves.evaluated_points_num()};
+    curves.ensure_can_interpolate_to_evaluated();

@@ -381,1 +388,4 @@
+    return eval;
   }
+
+  GArray src_buffer(src.type(), src.size());

@@ -381,1 +389,4 @@
   }
+
+  GArray src_buffer(src.type(), src.size());
+  src.materialize_to_uninitialized(src_buffer.data());