Cancellable rendering. Rendering in buckets of 48x48 pixels.

Also, add a preview pass, and spawn the rendering proper in a separate thread,
and the event loop in other (so the screen is responsive even while rendering).
Implemented a simple mesh.
Make Bitmap destructor virtual
Part II: Transformation (medium task).
Implement Ray->Mesh intersecion. Add some comments. Change the scene a bit.