bug#20607: 24.5; Problem with overlays at same position but in different windows

[Oleh Krehel]

Suppose I have two windows, WND1 and WND2, both displaying the same
buffer. In WND1, there's an overlay from 1 to 2 with window property
WND1 and display "ab". In WND2, there's an overlay from 1 to 3 with
window property WND2 and display "abc".

Instead of the expected, I see "ab" in WND1, but "abcabc" in WND2.  When
I `move-overlay' in the first window to 1-3, the glitch disappears, but
I actually want the overlay to be 1-2, not 1-3. If I have to make it
1-3, I'd have to modify the display as well to include one more
character from the buffer. I think it's better to fix the overlay logic
rather than go for this work-around.

Oleh