cursor-subsurface: new client

The cursor surface is usually pretty ordinary, e.g. a wl_surface with an attached wl_shm_buffer or a wl_surface without a buffer. The type of the client buffer may vary, but it's often an shm buffer in the wild.

But due to being a wl_surface, one could do more exotic things such as set a wp_viewport, or in case of this client, attach subsurfaces to the cursor surface.

If the compositor honors cursor subsurface trees, you should see a red translucent square (cursor_surface) and a green square (cursor_subsurface) centered over the bottom right corner of the red square.

example