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/DebugUI.h

@@ -6,6 +6,7 @@ class Window;
 struct RenderSettings;
 struct Scene;
 class Weather;
+class DayCycle;
 
 // Dear ImGui integration plus a ready-made tuning panel for the renderer.
 class DebugUI {
@@ -19,8 +20,9 @@ public:
 
     // Tweak panel for every look parameter (sun, sky, fog, AO, water, post).
     static void renderSettingsPanel(RenderSettings& settings, Scene& scene, float gpuMs);
-    // Rain amount, transition time and the storm preset.
-    static void weatherPanel(Weather& weather);
+    // Rain / overcast / fog / lightning, transition time and the weather
+    // presets; with `hour` an hour slider, with `day` its keyframes.
+    static void weatherPanel(Weather& weather, DayCycle* day = nullptr, float* hour = nullptr);
 
 private:
     bool initialized_ = false;