Add `-padding` argument

[ZeroMemes]

Arranges glyphs such that there are a # of empty pixels equal to value specified in the -padding argument on all sides.

-pxrange 5

-pxrange 5 -padding 5

[ZeroMemes]

On second thought, this approach is not foolproof and could cause issues with specific drop shadow and pixel range setups (which is what I intended to solve). The atlas and plane bounds need to be extended as well to allow the shadow offset texture coordinates to actually be sampled.