bugfix: clear selection when clicking above or below lines

bugfix: clear selection when clicking above or below lines

Matt Wynne pointed out that snap.love would crash when a node went off
screen. While debugging it I noticed that selection1 was being set when
it shouldn't be.

Turns out I introduced a bug when I fixed the inscript bug back in June
(commit 9656e137742). One invariant I want to preserve is: selection1
should be unset after a mouse click (press and release without
intervening drag). This invariant was violated in my bugfix back in
June. I was concerned only with selection back then, and I didn't
realize I was breaking the mouse click case (in a fairly subtle way; you
can have selection set, and when it's set identically to the cursor
everything looks the same).

I think there might still be an issue in snap.love after this fix. I
noticed screen_bottom1.pos was nil, and as far as I recall that should
never happen.