nanoemoji glyphmap.csv added additional column, breaks mkglyphmap.py

[anthrotype]

Heads up, in the latest nanoemoji we needed to modify the format of the glyphmap.csv file to accommodate for hybrid vector+bitmap color fonts (with both COLR, OT-SVG and CBDT tables), thus we added a column for the .PNG sources. The expected format is now

svg_file, png_file, glyph_name, codepoints*

Either the first or the second filenames can be left empty depending on whether it's vector-only, bitmap-only or hybrid.

[khaledhosny]

Thanks for the heads up. I’ll keep this in mind when updating the dependencies.