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/Camera.h

@@ -19,6 +19,10 @@ struct Camera {
     glm::vec3 right() const;    // horizontal right vector
     glm::mat4 view() const;
     glm::mat4 projection(float aspect) const;
+    // Projects `world` to window pixels (top-left origin) of a `viewportPx`
+    // viewport, using view() and projection(viewportPx.x / viewportPx.y).
+    // Returns false (outPx untouched) when the point is behind the camera.
+    bool worldToScreen(const glm::vec3& world, glm::vec2 viewportPx, glm::vec2& outPx) const;
 };
 
 }  // namespace hd2d