This allows to send wl_pointer.enter when switching between views
in a split/tabbed layout for instance. This (1) updates the cursor
image accordingly (2) makes it unnecessary to move the mouse before
scrolling. It's harmless to always call cursor_send_pointer_motion
because in case the focused surface hasn't changed this is a no-op.
The `last_focus != NULL` condition is required otherwise
cursor_send_pointer_motion will crash when sway starts up (the
sway_output doesn't yet have a workspace).
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