Correct bounds checking for design view highlight

The check should be for both x and y being less than 0, as when the
cursor is moved out of the design view "canvas" area, the values are set
to -1. Note this doesn't actually change behaviour, as the coordinates
are set to (-1, -1) whent the cursor leaves the area.

https://bugzilla.gnome.org/show_bug.cgi?id=723819