Use custom sprite sorting rules for vehicles diagonally under bridges

Extend bb sprite south for comparisons, but only for vehicles underneath
This is to avoid creating sprite sorting problems for vehicles
on top of the bridge

Adjust ParentSpriteToDraw struct

src/vehicle.cpp

@@ -1862,12 +1862,14 @@ static void DoDrawVehicle(const Vehicle *v)
 		}
 	}
 
+	ViewportSortableSpriteSpecialFlags special_flags = IsDiagonalDirection(v->direction) ? VSSF_NONE : VSSSF_SORT_SPECIAL | VSSSF_SORT_DIAG_VEH;
+
 	StartSpriteCombine();
 	for (uint i = 0; i < v->sprite_seq.count; ++i) {
 		PaletteID pal2 = v->sprite_seq.seq[i].pal;
 		if (!pal2 || (v->vehstatus & VS_CRASHED)) pal2 = pal;
 		AddSortableSpriteToDraw(v->sprite_seq.seq[i].sprite, pal2, v->x_pos + v->x_offs, v->y_pos + v->y_offs,
-			v->x_extent, v->y_extent, v->z_extent, v->z_pos, shadowed, v->x_bb_offs, v->y_bb_offs);
+			v->x_extent, v->y_extent, v->z_extent, v->z_pos, shadowed, v->x_bb_offs, v->y_bb_offs, 0, nullptr, special_flags);
 	}
 	EndSpriteCombine();
 }