add imgui library

3rdparty/imgui/examples/example_null/build_win32.bat

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+@REM Build for Visual Studio compiler. Run your copy of vcvars32.bat or vcvarsall.bat to setup command-line compiler.
+mkdir Debug
+cl /nologo /Zi /MD /I ..\.. *.cpp ..\..\*.cpp /FeDebug/example_null.exe /FoDebug/ /link gdi32.lib shell32.lib

3rdparty/imgui/examples/example_null/main.cpp

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+// dear imgui: null/dummy example application (compile and link imgui with no inputs, no outputs)
+#include "imgui.h"
+#include <stdio.h>
+
+int main(int, char**)
+{
+    IMGUI_CHECKVERSION();
+    ImGui::CreateContext();
+    ImGuiIO& io = ImGui::GetIO();
+
+    // Build atlas
+    unsigned char* tex_pixels = NULL;
+    int tex_w, tex_h;
+    io.Fonts->GetTexDataAsRGBA32(&tex_pixels, &tex_w, &tex_h);
+
+    for (int n = 0; n < 50; n++)
+    {
+        printf("NewFrame() %d\n", n);
+        io.DisplaySize = ImVec2(1920, 1080);
+        io.DeltaTime = 1.0f / 60.0f;
+        ImGui::NewFrame();
+
+        static float f = 0.0f;
+        ImGui::Text("Hello, world!");
+        ImGui::SliderFloat("float", &f, 0.0f, 1.0f);
+        ImGui::Text("Application average %.3f ms/frame (%.1f FPS)", 1000.0f / io.Framerate, io.Framerate);
+        ImGui::ShowDemoWindow(NULL);
+
+        ImGui::Render();
+    }
+
+    printf("DestroyContext()\n");
+    ImGui::DestroyContext();
+    return 0;
+}