Danielsson propagation in EdgeGrid::Grid::calculate_sdf() doubles a negative neighbor offset (-1 or -ncols) with a left shift, which is undefined behavior in C++17. Multiply by two instead.
Adds test_edge_grid.cpp, which checks signed distances inside and outside a rectangle beyond the initial edge band. Built with -fsanitize=shift, it aborts on the old code and passes with the fix.
