Fonts: Fix calling ClearTexData() (clearing CPU side font data) triggering an assert in NewFrame(). (#3487) + Backends: added additional assert to facilitate detecting user understand they haven't initialized a backend.

backends/imgui_impl_opengl2.cpp

@@ -96,6 +96,8 @@ void    ImGui_ImplOpenGL2_Shutdown()
 void    ImGui_ImplOpenGL2_NewFrame()
 {
     ImGui_ImplOpenGL2_Data* bd = ImGui_ImplOpenGL2_GetBackendData();
+    IM_ASSERT(bd != NULL && "Did you call ImGui_ImplOpenGL2_Init()?");
+
     if (!bd->FontTexture)
         ImGui_ImplOpenGL2_CreateDeviceObjects();
 }