ENH: improve archor of internal bridge

Add loop inside sparse infill of lower layer
to provide archor for internal bridge.

This option controls how many lower layers generate
loop path.

Signed-off-by: salt.wei <salt.wei@bambulab.com>
Change-Id: Ifdb6db9090115732aa2cacf5d155b587a10e18fc
(cherry picked from commit 4fdb87ecd5c2e680f6bad2d018ba39cd05f787ce)

src/libslic3r/Surface.hpp

@@ -15,6 +15,8 @@ enum SurfaceType {
     stBottomBridge,
     // Normal sparse infill.
     stInternal,
+    // Normal sparse infill.
+    stInternalWithLoop,
     // Full infill, supporting the top surfaces and/or defining the verticall wall thickness.
     stInternalSolid,
     // 1st layer of dense infill over sparse infill, printed with a bridging extrusion flow.