add opengl 2.1 support, update imgui, update premake script

external/imgui/imgui_impl_glfw_gl2.h

@@ -0,0 +1,32 @@
+// ImGui GLFW binding with OpenGL (legacy, fixed pipeline)
+// (GLFW is a cross-platform general purpose library for handling windows, inputs, OpenGL/Vulkan graphics context creation, etc.)
+
+// Implemented features:
+//  [X] User texture binding. Cast 'GLuint' OpenGL texture identifier as void*/ImTextureID. Read the FAQ about ImTextureID in imgui.cpp.
+
+// **DO NOT USE THIS CODE IF YOUR CODE/ENGINE IS USING MODERN OPENGL (SHADERS, VBO, VAO, etc.)**
+// **Prefer using the code in the opengl3_example/ folder**
+// See imgui_impl_glfw.cpp for details.
+
+// You can copy and use unmodified imgui_impl_* files in your project. See main.cpp for an example of using this.
+// If you use this binding you'll need to call 4 functions: ImGui_ImplXXXX_Init(), ImGui_ImplXXXX_NewFrame(), ImGui::Render() and ImGui_ImplXXXX_Shutdown().
+// If you are new to ImGui, see examples/README.txt and documentation at the top of imgui.cpp.
+// https://github.com/ocornut/imgui
+
+struct GLFWwindow;
+
+IMGUI_API bool        ImGui_ImplGlfwGL2_Init(GLFWwindow* window, bool install_callbacks);
+IMGUI_API void        ImGui_ImplGlfwGL2_Shutdown();
+IMGUI_API void        ImGui_ImplGlfwGL2_NewFrame();
+IMGUI_API void        ImGui_ImplGlfwGL2_RenderDrawData(ImDrawData* draw_data);
+
+// Use if you want to reset your rendering device without losing ImGui state.
+IMGUI_API void        ImGui_ImplGlfwGL2_InvalidateDeviceObjects();
+IMGUI_API bool        ImGui_ImplGlfwGL2_CreateDeviceObjects();
+
+// GLFW callbacks (registered by default to GLFW if you enable 'install_callbacks' during initialization)
+// Provided here if you want to chain callbacks yourself. You may also handle inputs yourself and use those as a reference.
+IMGUI_API void        ImGui_ImplGlfw_MouseButtonCallback(GLFWwindow* window, int button, int action, int mods);
+IMGUI_API void        ImGui_ImplGlfw_ScrollCallback(GLFWwindow* window, double xoffset, double yoffset);
+IMGUI_API void        ImGui_ImplGlfw_KeyCallback(GLFWwindow* window, int key, int scancode, int action, int mods);
+IMGUI_API void        ImGui_ImplGlfw_CharCallback(GLFWwindow* window, unsigned int c);