Added a way to specify amount of padding for each glyph

[maatko]

Allows you to specify the amount of padding used in the atlas packer when packing the glyphs,
this can be useful when drawing the glyph in the fragment shader for all sorts of effects

[znakeeye]

How come this PR was not merged? Having padding between the glyphs allows you to seamlessly render e.g. shadow without reading from the adjacent characters. I.e. Without padding, this shadron script (see line 36) will produce artifacts. (At least when put that code in a regular shader.)

[Chlumsky]

I am not opposed to a padding option but it has to be implemented correctly, which this merge request does not do. It is not as simple as it might initially seem. I am planning to do a larger update to this library in the first quarter of this year, so I will try to add a padding option as well.