add backend_test for sdl2 opengl3

backend_test/cimgui_extras.h

@@ -0,0 +1,11 @@
+
+
+// GL3W/GLFW
+#include <GL/gl3w.h>    // This example is using gl3w to access OpenGL functions (because it is small). You may use glew/glad/glLoadGen/etc. whatever already works for you.
+
+//making it accesible for luajit
+#ifdef _WIN32
+__declspec( dllexport ) int Do_gl3wInit(void);
+#else
+int Do_gl3wInit(void);
+#endif