Sprite sheets, atlases, game UI, time of day, weather presets, headless captures

- BillboardInstance: per-instance uv sub-rect, feet anchor, turn, dithered alpha, flash;
  BillboardBatch::sharedTexture; Texture createEmpty/update/cellUV/cellAnchor/nearest
- SpriteAtlas: fixed-cell runtime atlas with gutters
- hd2d::ui: Octopath-style panels, fonts (Cinzel, EB Garamond), dialogue box with
  typewriter + portraits + choices, menus, nameplates, toasts, ImGui theme
- DayCycle (dawn/day/dusk/night keyframes, sun + moon arcs), Weather overcast/mist/
  lightning, night-aware presets, stars and moon in the sky
- Hidden-window offscreen captures (Window visible=false, Renderer output FBO,
  OffscreenTarget); --screenshot never opens a window
- Camera::worldToScreen; CMake usable via add_subdirectory (HD2D_BUILD_DEMO,
  HD2D_ENGINE_ASSET_DIR)

engine/include/hd2d/Window.h

@@ -10,19 +10,27 @@ namespace hd2d {
 class Window {
 public:
     // Non-resizable windows float on tiling WMs, keeping an exact size.
-    bool create(int width, int height, const std::string& title, bool vsync = true, bool resizable = true);
+    // `visible = false` creates a hidden, never-focused window that only
+    // provides the GL context (headless captures: render into an
+    // OffscreenTarget, see Screenshot.h). Hidden windows ignore vsync,
+    // swapBuffers() is a no-op (unmapped Wayland surfaces never get frame
+    // callbacks, so a real swap could block forever) and every input query
+    // reports nothing, so the user's keyboard/mouse cannot affect them.
+    bool create(int width, int height, const std::string& title, bool vsync = true, bool resizable = true,
+                bool visible = true);
     void destroy();
 
     // Polls events and refreshes per-frame input state. Returns false once the
     // window should close.
     bool pollEvents();
     void swapBuffers();
+    bool visible() const { return visible_; }
 
     bool keyDown(int glfwKey) const;
     bool keyPressed(int glfwKey) const;  // went down this frame
     bool mouseDown(int glfwButton) const;
-    glm::vec2 mouseDelta() const { return mouseDelta_; }
-    float scrollDelta() const { return scrollDelta_; }
+    glm::vec2 mouseDelta() const { return visible_ ? mouseDelta_ : glm::vec2(0.0f); }
+    float scrollDelta() const { return visible_ ? scrollDelta_ : 0.0f; }
 
     int width() const { return width_; }
     int height() const { return height_; }
@@ -38,6 +46,7 @@ private:
     GLFWwindow* window_ = nullptr;
     int width_ = 0, height_ = 0;
     bool resized_ = false;
+    bool visible_ = true;
     bool keys_[512] = {};
     bool prevKeys_[512] = {};
     glm::vec2 lastMouse_{0.0f};