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>

CMakeLists.txt

@@ -30,6 +30,7 @@ add_library(effector_core STATIC
     src/effects/blur.cpp
     src/effects/grain.cpp
     src/effects/monochrome.cpp
+    src/effects/overlay.cpp
     src/effects/scanlines.cpp
 )
 target_include_directories(effector_core PUBLIC src)