[Test] On-desktop objects

A quick demo for on-desktop objects in Godot 4.5

In the demo, a full-screen application will overlay the desktop, and by detecting per-pixel transparency, the window properties change between clickable/passthrough states, allowing anything behind to be interacted with instead

This is useful if you want to make an overlay game like Outcore but need control over pixels instead of collisions (for examples, if you want 3D objects, and you want an alternative to raycasting)

The source code is available on Github for reference