Fix #130376: GPv3: Outline modifier traces flat caps incorrectly

For flat caps, only one point is added in data.positions array for
endpoints of stroke. It results in incomplete outline. To resolve
this, add another point in new curves at the exact opposite position.

Pull Request: https://projects.blender.org/blender/blender/pulls/130437

source/blender/editors/grease_pencil/intern/grease_pencil_geom.cc

@@ -503,6 +503,8 @@ static void generate_cap(const float3 &point,
                                        r_src_indices);
       break;
     case GP_STROKE_CAP_FLAT:
+      r_perimeter.append(point - normal * radius);
+      r_src_indices.append(src_point_index);
       r_perimeter.append(point + normal * radius);
       r_src_indices.append(src_point_index);
       break;