Export pointer visibility

[Julien Danjou]

Hi there,

I've played with mouse-avoidance-mode today, but noticed that it did
move the cursor even if it was hidden by `make-pointer-invisible'.

So there's 2 patches, one exporting the pointer visibility and another
one enhancing avoid.el to do nothing when the pointer is already
invisible.

Feedbacks welcome,

-- 
Julien Danjou
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