after a while, loupe occassionally fails to disappear when leaving the image

[nishanths]

To reproduce: Repeatedly hover over and out of the target image. In most cases the loupe disappears as expected. After a while, in some cases, the loupe remains visible even when the mouse is moved out of the image.

[snowfluke]

I think it is because there's a race, either we remove the event listener first or we recalculate the exit condition

[snowfluke]

the exitDistance should be half of the width for x and half of the height for y from the magnifier glass

[snowfluke]

Just submitted a new PR @nishanths