Add 1 pixel padding to generated font atlas

This avoids artifacts when using SDF fonts with some characters that
happen to be on the first column of the texture atlas.

Example: https://www.dropbox.com/s/6t6edr0w4enyvdm/Screenshot%202015-01-01%2016.55.02.png?dl=0 (left side of p, 7 and K is not smooth)

Fixed: https://www.dropbox.com/s/w4ha6824j6j8s15/Screenshot%202015-01-01%2016.55.51.png?dl=0