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/Texture.h

@@ -19,9 +19,14 @@ bool loadImage(const std::string& path, Image& out);
 // Decodes PNG bytes from memory. glTF images use flipY = false (their UV
 // origin is top-left, which matches GL when rows are uploaded unflipped).
 bool decodeImage(const unsigned char* data, size_t size, Image& out, bool flipY);
+// Replaces the colors of fully transparent pixels with their nearest opaque
+// neighbour (loadImage/decodeImage already do this). Call it on images you
+// compose yourself so filtering never bleeds dark fringes into edges.
+void bleedTransparentColors(Image& img);
 
 // sRGB pixel-art texture. Sampled with "sharp bilinear" in shaders, so it uses
-// linear filtering with mipmaps rather than GL_NEAREST.
+// linear filtering with mipmaps rather than GL_NEAREST. `nearest` = GL_NEAREST
+// without mips instead (crisp ImGui previews of pixel art).
 class Texture {
 public:
     Texture() = default;
@@ -31,8 +36,14 @@ public:
     Texture& operator=(Texture&& o) noexcept;
     ~Texture();
 
-    bool load(const std::string& path, bool repeat = false);
-    bool create(const Image& image, bool repeat = false, float anisotropy = 1.0f);
+    bool load(const std::string& path, bool repeat = false, bool nearest = false);
+    bool create(const Image& image, bool repeat = false, float anisotropy = 1.0f, bool nearest = false);
+    // Fully transparent w x h texture (sRGB RGBA8, mipmapped, clamped) to fill with update().
+    bool createEmpty(int w, int h);
+    // Uploads `img` with its bottom-left pixel at (x, y) (GL orientation,
+    // y = 0 is the bottom row) and regenerates the mips. Does not change
+    // opaqueBounds(), which always describes the image given to create().
+    void update(int x, int y, const Image& img);
     void bind(GLuint unit) const { glBindTextureUnit(unit, id_); }
 
     GLuint id() const { return id_; }
@@ -41,9 +52,16 @@ public:
     // Billboards use minY to plant the visible feet on the ground.
     glm::vec4 opaqueBounds() const { return opaqueBounds_; }
 
+    // Grid sprite sheets authored top-down (row 0 = top row of the PNG), with
+    // `cell` x `cell` pixel frames. cellUV is a BillboardInstance::uv rect;
+    // cellAnchor is a BillboardInstance::anchor: the v of the bottom edge of
+    // pixel row `feetRowFromCellTop` (0 = the cell's top row) of that cell.
+    static glm::vec4 cellUV(glm::ivec2 texSize, glm::ivec2 cell, int col, int rowFromTop);
+    static float cellAnchor(glm::ivec2 texSize, glm::ivec2 cell, int rowFromTop, int feetRowFromCellTop);
+
 private:
     GLuint id_ = 0;
-    int width_ = 0, height_ = 0;
+    int width_ = 0, height_ = 0, levels_ = 0;
     glm::vec4 opaqueBounds_{0, 0, 1, 1};
 };