gl: Prevent border color bleeding in image atlas

In a normal image, sampling takes at a center of a pixel. If samples happens
around border, the sampling will uses the border color instead of
reaching beyond border.  However, with atlas, "image border" is not
the atlas border.  This becomes problematic bcause sampling at "image
border" reaches beyond border.  To avoid this problem, we inset the "image
border" by 0.5 so that sampling always occur inside desired image.