添加 LightGame 关卡编辑器并修复死亡扣命、光感门阈值与渲染问题

- 新增 LevelEditor：ImGui 面板支持 tile 绘制、对象放置、选择、擦除、网格/碰撞体可视化、关卡导出为头文件
- 修复 LevelRenderer::draw 中 draw_tilemap 错误传入负 screen 坐标导致摄像机移动后 tile 消失
- 修复 Preview Light 滑块用 reinterpret_cast 将 uint16_t 强转为 int* 写入 4 字节覆盖相邻 bool 成员
- 修复编辑器放置光感门缺少 light_threshold 默认值导致不响应亮度变化
- 统一 LightPlatform[3192,4095] ShadowPlatform[0,1023] Door[1023,3192] 阈值到关卡数据与编辑器默认值
- 修复 SDL_StopTextInput 导致 ImGui InputText 无法输入及键盘状态异常
- 修复死亡状态每帧扣命导致一次死亡直接 GameOver
- 集成 ImGui (SDL2 Renderer 后端) 到构建系统

third_party/imgui/backends/imgui_impl_sdl2.h

@@ -0,0 +1,46 @@
+// dear imgui: Platform Backend for SDL2
+// This needs to be used along with a Renderer (e.g. DirectX11, OpenGL3, Vulkan..)
+// (Info: SDL2 is a cross-platform general purpose library for handling windows, inputs, graphics context creation, etc.)
+
+// Implemented features:
+//  [X] Platform: Clipboard support.
+//  [X] Platform: Mouse support. Can discriminate Mouse/TouchScreen.
+//  [X] Platform: Keyboard support. Since 1.87 we are using the io.AddKeyEvent() function. Pass ImGuiKey values to all key functions e.g. ImGui::IsKeyPressed(ImGuiKey_Space). [Legacy SDL_SCANCODE_* values will also be supported unless IMGUI_DISABLE_OBSOLETE_KEYIO is set]
+//  [X] Platform: Gamepad support. Enabled with 'io.ConfigFlags |= ImGuiConfigFlags_NavEnableGamepad'.
+//  [X] Platform: Mouse cursor shape and visibility. Disable with 'io.ConfigFlags |= ImGuiConfigFlags_NoMouseCursorChange'.
+//  [X] Platform: Basic IME support. App needs to call 'SDL_SetHint(SDL_HINT_IME_SHOW_UI, "1");' before SDL_CreateWindow()!.
+
+// You can use unmodified imgui_impl_* files in your project. See examples/ folder for examples of using this.
+// Prefer including the entire imgui/ repository into your project (either as a copy or as a submodule), and only build the backends you need.
+// Learn about Dear ImGui:
+// - FAQ                  https://dearimgui.com/faq
+// - Getting Started      https://dearimgui.com/getting-started
+// - Documentation        https://dearimgui.com/docs (same as your local docs/ folder).
+// - Introduction, links and more at the top of imgui.cpp
+
+#pragma once
+#include "imgui.h"      // IMGUI_IMPL_API
+#ifndef IMGUI_DISABLE
+
+struct SDL_Window;
+struct SDL_Renderer;
+struct _SDL_GameController;
+typedef union SDL_Event SDL_Event;
+
+// Follow "Getting Started" link and check examples/ folder to learn about using backends!
+IMGUI_IMPL_API bool     ImGui_ImplSDL2_InitForOpenGL(SDL_Window* window, void* sdl_gl_context);
+IMGUI_IMPL_API bool     ImGui_ImplSDL2_InitForVulkan(SDL_Window* window);
+IMGUI_IMPL_API bool     ImGui_ImplSDL2_InitForD3D(SDL_Window* window);
+IMGUI_IMPL_API bool     ImGui_ImplSDL2_InitForMetal(SDL_Window* window);
+IMGUI_IMPL_API bool     ImGui_ImplSDL2_InitForSDLRenderer(SDL_Window* window, SDL_Renderer* renderer);
+IMGUI_IMPL_API bool     ImGui_ImplSDL2_InitForOther(SDL_Window* window);
+IMGUI_IMPL_API void     ImGui_ImplSDL2_Shutdown();
+IMGUI_IMPL_API void     ImGui_ImplSDL2_NewFrame();
+IMGUI_IMPL_API bool     ImGui_ImplSDL2_ProcessEvent(const SDL_Event* event);
+
+// Gamepad selection automatically starts in AutoFirst mode, picking first available SDL_Gamepad. You may override this.
+// When using manual mode, caller is responsible for opening/closing gamepad.
+enum ImGui_ImplSDL2_GamepadMode { ImGui_ImplSDL2_GamepadMode_AutoFirst, ImGui_ImplSDL2_GamepadMode_AutoAll, ImGui_ImplSDL2_GamepadMode_Manual };
+IMGUI_IMPL_API void     ImGui_ImplSDL2_SetGamepadMode(ImGui_ImplSDL2_GamepadMode mode, struct _SDL_GameController** manual_gamepads_array = NULL, int manual_gamepads_count = -1);
+
+#endif // #ifndef IMGUI_DISABLE