Avoid signed-shift undefined behavior in EdgeGrid distance propagation (#16626)

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.

tests/libslic3r/CMakeLists.txt

@@ -35,6 +35,7 @@ add_executable(${_TEST_NAME}_tests
     test_preset_diff.cpp
     test_vendor_cache.cpp
     test_preset_options.cpp
+    test_edge_grid.cpp
     test_elephant_foot_compensation.cpp
     test_fill_corner_smoothing.cpp
     test_filament_mixer.cpp

tests/libslic3r/test_edge_grid.cpp

@@ -0,0 +1,33 @@
+#include <catch2/catch_test_macros.hpp>
+#include <catch2/matchers/catch_matchers_floating_point.hpp>
+#include "catch2/matchers/catch_matchers.hpp"
+#include "libslic3r/BoundingBox.hpp"
+#include "libslic3r/EdgeGrid.hpp"
+#include "libslic3r/Point.hpp"
+#include "libslic3r/Polygon.hpp"
+#include "libslic3r/libslic3r.h"
+
+using namespace Slic3r;
+
+TEST_CASE("EdgeGrid propagates signed distances across the interior and exterior", "[EdgeGrid][Regression]")
+{
+    const Polygon rectangle {Point::new_scale(-10., -12.), Point::new_scale(10., -12.),
+                             Point::new_scale(10., 12.), Point::new_scale(-10., 12.)};
+    const Polygons outlines {rectangle};
+    const BoundingBox bounds(Point::new_scale(-30., -32.), Point::new_scale(30., 32.));
+    EdgeGrid::Grid grid(bounds);
+    grid.create(outlines, scale_(1.));
+    // Distances beyond the initialized edge band require backward propagation.
+    // With -fsanitize=shift, this also checks negative neighbour offsets.
+    grid.calculate_sdf();
+    const auto distance = [&](double x, double y) {
+        return grid.signed_distance_bilinear(Point::new_scale(x, y)) * SCALING_FACTOR;
+    };
+    REQUIRE_THAT(distance(0., 0.), Catch::Matchers::WithinAbs(-10., 0.01));
+    for (double sign : {-1., 1.}) {
+        REQUIRE_THAT(distance(sign * 5., 0.), Catch::Matchers::WithinAbs(-5., 0.01));
+        REQUIRE_THAT(distance(0., sign * 7.), Catch::Matchers::WithinAbs(-5., 0.01));
+        REQUIRE_THAT(distance(sign * 20., 0.), Catch::Matchers::WithinAbs(10., 0.01));
+        REQUIRE_THAT(distance(0., sign * 22.), Catch::Matchers::WithinAbs(10., 0.01));
+    }
+}