Got Mac focus fixed up.

I still don't know why calling setCursorPosition() on regaining focus
is broken, but I've just worked around it. Remember the calculcated
point beyond one frame- continue using it until we get a genuine
mouse moved event.
Bounds-checking no longer required.

Since we're not repositioning the cursor manually, we don't need
to ensure that the mouse is in-bounds when we received spurious
refocusings (which I also might only be seeing with my Terminals
set up with focus-follows-mouse).
Cleanup, optimization, commenting.
Renamed SLOPPY_FOCUS to NAIVE_FOCUS, which is more accurate.

Still enabled.