Testcase and fix for a problem transforming an empty bbox with very small numbers in a transformation matrix

src/linalg.cc

@@ -14,6 +14,7 @@
 */
 BoundingBox operator*(const Transform3d &m, const BoundingBox &box)
 {
+	if (box.isEmpty()) return box;
 	BoundingBox newbox;
 	Vector3d boxvec[2] = { box.min(), box.max() };
 	for (int k=0;k<2;k++) {

testdata/scad/misc/rotate-empty-bbox.scad

@@ -0,0 +1,11 @@
+// rotate([90,0,0]) results in a matrix with some close-to-zero values
+// Transforming an empty boundingbox by this matrix has caused a problem resulting
+// in the union being evaluated to nothing.
+rotate([90,0,0]) 
+difference() {
+  cube(60, center=true);
+  union() {
+    translate([0,0,40]) cube(50, center=true);
+    cube(0);
+  }
+}

tests/CMakeLists.txt

@@ -840,7 +840,8 @@ list(APPEND CGALPNGTEST_FILES ${CMAKE_SOURCE_DIR}/../testdata/scad/misc/include-
                            ${CMAKE_SOURCE_DIR}/../testdata/scad/misc/use-tests.scad
                            ${CMAKE_SOURCE_DIR}/../testdata/scad/bugs/transform-nan-inf-tests.scad
                            ${CMAKE_SOURCE_DIR}/../testdata/scad/misc/localfiles-test.scad
-                           ${CMAKE_SOURCE_DIR}/../testdata/scad/misc/localfiles_dir/localfiles-compatibility-test.scad)
+                           ${CMAKE_SOURCE_DIR}/../testdata/scad/misc/localfiles_dir/localfiles-compatibility-test.scad
+                           ${CMAKE_SOURCE_DIR}/../testdata/scad/misc/rotate-empty-bbox.scad)
 
 list(APPEND OPENCSGTEST_FILES ${CGALPNGTEST_FILES})
 list(APPEND OPENCSGTEST_FILES ${CMAKE_SOURCE_DIR}/../testdata/scad/bugs/intersection-prune-test.scad)