Expose the SDF function separately so that it can be used without canvassing text

There are precious few SDF libraries out there, and it would be quite valuable if there was a way to import just the SDF parts, for when you already _have_ a pixel matrix as uint8array, and you don't need to draw anything to a canvas, you just need the SDF for that pixel matrix.

Start by locating the repository's SDF generation and canvas/text entry points. Determine how the SDF logic can accept an existing uint8array without canvas work, then verify that the SDF functionality is separately importable and still supports the existing font-generation path.

Written by the indexing model from the issue text.