Add an image overlay effect

Lays a single picture from disk over its input: fitted, filled, stretched,
left at its own size or tiled, centred or pinned to the top left corner,
nudged by an offset and mixed in by its opacity.

The file it needs is a new kind of param, Path, so the interface shows a
chooser and the CLI takes a file name, both without knowing an overlay
exists. Projects write it relative to their own folder, as they do a
source, and Image::load can now hand back the alpha channel instead of
flattening it onto white, so a cut-out PNG keeps its cut-out.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/core/image.hpp

@@ -34,7 +34,11 @@ public:
 
     // Decodes via GdkPixbuf (PNG, JPEG, WebP, TIFF, BMP, ...), applying the
     // embedded EXIF orientation. Throws std::runtime_error on failure.
-    static Image load(const std::string& path);
+    // Transparency is flattened onto white, as a viewer would show it, unless
+    // `alphaOut` is given: it then receives one byte per pixel (255 throughout
+    // for a file without an alpha channel) and the colours are left untouched,
+    // which is what an effect compositing the image needs.
+    static Image load(const std::string& path, std::vector<std::uint8_t>* alphaOut = nullptr);
 
     // Encodes by file extension; png is used when the extension is unknown.
     void save(const std::string& path) const;