Split image2.cpp into two files to help speed up the build.

tools/python/CMakeLists.txt

@@ -49,6 +49,7 @@ set(python_srcs
    src/image.cpp
    src/image2.cpp
    src/image3.cpp
+   src/image4.cpp
    src/rectangles.cpp
    src/object_detection.cpp
    src/shape_predictor.cpp

tools/python/src/dlib.cpp

@@ -21,6 +21,7 @@ void bind_svm_struct(py::module& m);
 void bind_image_classes(py::module& m);
 void bind_image_classes2(py::module& m);
 void bind_image_classes3(py::module& m);
+void bind_image_classes4(py::module& m);
 void bind_rectangles(py::module& m);
 void bind_object_detection(py::module& m);
 void bind_shape_predictors(py::module& m);
@@ -98,6 +99,7 @@ PYBIND11_MODULE(dlib, m)
     bind_image_classes(m);
     bind_image_classes2(m);
     bind_image_classes3(m);
+    bind_image_classes4(m);
     bind_object_detection(m);
     bind_shape_predictors(m);
     bind_correlation_tracker(m);