Fix quad vertices for ImageOverlay and TextOverlay

The quad coords were off-by-one - a 2x2 quad should span from, for example, `(0, 0)` to `(2, 2)`, but instead it would span the pixel coordinates `(0, 0)` to `(1, 1)`.

Old: https://www.dropbox.com/s/jgfvzz73d5tckh1/Screenshot%202014-12-19%2015.02.12.png?dl=0
New: https://www.dropbox.com/s/fs3uzwyicwu1i2m/Screenshot%202014-12-19%2015.01.47.png?dl=0