Enabled the C++ fillers for all infills, not just the supports.

Made sure the C++ fillers are instantiated at the worker threads,
where there are being released.
Extended the FillRectilinear2 to calculate the contour / line intersection
with exact arithmetics, improved robustness and added error handling
and error reporting, if the contours to be filled are not correct.

xs/src/libslic3r/libslic3r.h

@@ -6,6 +6,7 @@
 #include <iostream>
 #include <sstream>
 #include <stdint.h>
+#include <stdarg.h>
 
 #define SLIC3R_VERSION "1.3.0-dev"
 
@@ -63,6 +64,18 @@ void confess_at(const char *file, int line, const char *func, const char *pat, .
 	#define SLIC3R_CPPVER 0
 #endif
 
+#define DEBUG_FILE_PREFIX "out/"
+
+inline std::string debug_out_path(const char *name, ...)
+{
+	char buffer[2048];
+	va_list args;
+	va_start(args, name);
+	vsprintf(buffer, name, args);
+	va_end(args);
+	return std::string(DEBUG_FILE_PREFIX) + std::string(buffer);
+}
+
 // Write slices as SVG images into out directory during the 2D processing of the slices.
 // #define SLIC3R_DEBUG_SLICE_PROCESSING