Don't draw cell backgrounds on half pixels on non-retina devices

On non-retina devices the shadows/border overlapping thing for cell backgrounds is pretty blurry and sticks out near our card-styled (but not drawn using cell backgrounds) views, and also makes it difficult to line our views up and have them look correct - the half-white/half-shadow edge makes the card look bigger than it is. On retina we don't have any such problem though, so half-pixels are ok there.

Provides a minPixelOffset so that people can bail out to the old behaviour if they so choose.