chore: add imgui library

third_party/imgui/CMakeLists.txt

@@ -0,0 +1,23 @@
+cmake_minimum_required(VERSION 3.15)
+
+project(imui)
+
+set(IMGUI_SOURCES
+    imgui.cpp
+    imgui_draw.cpp
+    imgui_tables.cpp
+    imgui_widgets.cpp
+    imgui_impl_glfw.cpp
+    imgui_impl_opengl3.cpp
+)
+
+add_library(imgui STATIC ${IMGUI_SOURCES})
+
+target_include_directories(imgui PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})
+
+target_link_libraries(
+    imgui
+    PUBLIC
+    glfw
+    OpenGL::GL
+)