Properly orient mirrored blend mesh faces

If the mirrored axis count is odd, the face orientation has logically
been inverted.  In such cases, the winding order should be inverted as
well, easing the burden on applications sensitive to index ordering
(e.g., one-sided renderers) to detect and fix such issues after import.
Add this functionality for .blend meshes.

code/BlenderModifier.cpp

@@ -248,6 +248,15 @@ void  BlenderModifier_Mirror :: DoIt(aiNode& out, ConversionData& conv_data,  co
 			}
 		}
 
+		// Only reverse the winding order if an odd number of axes were mirrored.
+		if (xs * ys * zs < 0) {
+			for( unsigned int i = 0; i < mesh->mNumFaces; i++) {
+				aiFace& face = mesh->mFaces[i];
+				for( unsigned int fi = 0; fi < face.mNumIndices / 2; ++fi)
+					std::swap( face.mIndices[fi], face.mIndices[face.mNumIndices - 1 - fi]);
+			}
+		}
+
 		conv_data.meshes->push_back(mesh);
 	}
 	unsigned int* nind = new unsigned int[out.mNumMeshes*2];