Enhance Bitmap class with error handling and code improvements

- Introduced BitmapError class for more specific error handling in the Bitmap class.
- Updated data checks to use `empty()` for clarity and consistency.
- Improved pixel data handling by ensuring proper type usage and modifying loop constructs for better performance.
- Refactored constructor and method implementations for improved readability and maintainability.

src/app/gfx/bitmap.h

@@ -151,7 +151,7 @@ class Bitmap {
   int width() const { return width_; }
   int height() const { return height_; }
   int depth() const { return depth_; }
-  int size() const { return data_.size(); }
+  auto size() const { return data_.size(); }
   const uint8_t *data() const { return data_.data(); }
   std::vector<uint8_t> &mutable_data() { return data_; }
   SDL_Surface *surface() const { return surface_; }