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This lets the server discard requests received after the pointer has
exited and the re-entered a surface.

Well, maybe not support, but we at least recognize touchpads and touchscreens
and try to work with them.

We always call this between window_draw() and attaching the window surface
so the surface we're copying to isn't visible.

And provide a function to write it to a connection.

Now wl_client_post_event() is the only entry point for sending out events.

Avoids the malloc+copy, and is a step towards using the closure for
marshalling too.

This way we can invoke it multiple times without having to demarshal
and build the cfi every time.

Change the cursor in response to target events.