New parameter "Slicing Mode" for supporting 3DLabPrint airplane models.

S3D's strategy for merging self intersecting models is "Even / Odd"
which PrusaSlicer now supports as an alternative to "Positive" rule.
Also added a "Close Holes" option to fill in all internal structures.
3D-Labprint Models aren't sliceable (till years) #3062 #3708

src/libslic3r/TriangleMeshSlicer.hpp

@@ -12,8 +12,13 @@ struct MeshSlicingParams
 {
     enum class SlicingMode : uint32_t {
         // Regular slicing, maintain all contours and their orientation.
+        // slice_mesh_ex() applies ClipperLib::pftPositive rule to the result of slice_mesh().
         Regular,
-        // Maintain all contours, orient all contours CCW, therefore all holes are being closed.
+        // For slicing 3DLabPrints plane models (aka to be compatible with S3D default strategy).
+        // slice_mesh_ex() applies ClipperLib::pftEvenOdd rule. slice_mesh() slices EvenOdd as Regular.
+        EvenOdd,
+        // Maintain all contours, orient all contours CCW.
+        // slice_mesh_ex() applies ClipperLib::pftPositive rule, thus holes will be closed.
         Positive,
         // Orient all contours CCW and keep only the contour with the largest area.
         // This mode is useful for slicing complex objects in vase mode.