cursor.c: use subsurface as reference for out-of-surface movement

cursor.c: use subsurface as reference for out-of-surface movement

The protocol states that the wl_pointer motion coordinates must be
relative to the focused surface (e.g. the surface that last received
a wl_pointer enter event).

Before this patch, the coordinates were relative to the toplevel
surface instead, resulting in subsurface events having the wrong
coordinates when pressing a button over a subsurface and moving
the cursor outside of that subsurface.

Fixes: #2542