Fix null char reading and backslash separated multi lines

Stream_support/include/CGAL/IO/OBJ.h

@@ -25,6 +25,7 @@
 #include <boost/utility/enable_if.hpp>
 #include <CGAL/boost/graph/Named_function_parameters.h>
 
+#include <algorithm>
 #include <fstream>
 #include <iostream>
 #include <sstream>
@@ -72,12 +73,30 @@ bool read_OBJ(std::istream& is,
   bool tex_found(false), norm_found(false);
   while(getline(is, line))
   {
-    if(line.empty())
-      continue;
+    // get last non-whitespace, non-null character
+    auto last = std::find_if(line.rbegin(), line.rend(), [](char c) { return c != '\0' && !std::isspace(c); });
+    if(last == line.rend())
+      continue; // line is empty or only whitespace
+
+    // keep reading lines as long as the last non-whitespace, non-null character is a backslash
+    while(last != line.rend() && *last == '\\')
+    {
+      // remove everything from the backslash (included)
+      line = line.substr(0, line.size() - (last - line.rbegin()) - 1);
+
+      std::string next_line;
+      if(!getline(is, next_line))
+        break;
+
+      line += next_line;
+      last = std::find_if(line.rbegin(), line.rend(), [](char c) { return c != '\0' && !std::isspace(c); });
+    }
+
+    CGAL_assertion(!line.empty());
 
     std::istringstream iss(line);
     if(!(iss >> s))
-      continue; // can't read anything on the line, whitespace only?
+      continue;
 
     if(s == "v")
     {