Fix T72729: Mask Modifier Loose Edges Not Shown

When using the mask modifier loose edges could be added to the mesh.
These edges weren't marked as loose edges and wasn't picked up by other
areas of blender.

This fix recalculates the loose edges so they have the correct flag
`ME_LOOSE_EDGE`.

Reviewed By: Sybren Stüvel

Differential Revision: https://developer.blender.org/D7766

source/blender/modifiers/intern/MOD_mask.cc

@@ -366,6 +366,7 @@ static Mesh *applyModifier(ModifierData *md, const ModifierEvalContext *ctx, Mes
   copy_masked_polys_to_new_mesh(
       *mesh, *result, vertex_map, edge_map, masked_poly_indices, new_loop_starts);
 
+  BKE_mesh_calc_edges_loose(result);
   /* Tag to recalculate normals later. */
   result->runtime.cd_dirty_vert |= CD_MASK_NORMAL;