Fix T51700: OBJ exporter would have inverted normals in some cases.

Applying a matrix with non-uniform negative scaling to a mesh requires
normals (face winfing) to be inverted back to their original orientation!

io_scene_obj/__init__.py

@@ -21,7 +21,7 @@
 bl_info = {
     "name": "Wavefront OBJ format",
     "author": "Campbell Barton, Bastien Montagne",
-    "version": (2, 3, 3),
+    "version": (2, 3, 4),
     "blender": (2, 77, 0),
     "location": "File > Import-Export",
     "description": "Import-Export OBJ, Import OBJ mesh, UV's, materials and textures",

io_scene_obj/export_obj.py

@@ -397,6 +397,9 @@ def write_file(filepath, objects, scene,
                             continue
 
                         me.transform(EXPORT_GLOBAL_MATRIX * ob_mat)
+                        # If negative scaling, we have to invert the normals...
+                        if ob_mat.determinant() < 0.0:
+                            me.flip_normals()
 
                         if EXPORT_TRI:
                             # _must_ do this first since it re-allocs arrays