Support for combining marks in UnifontPrintBuffer

[mike42]

Currently UnifontPrintBuffer cannot print eg. Khmer or Thai, because the combining characters are shown as separate glyphs.

Unifont ships with a file called plane00-combining.txt, which shows the x-offset of the combining character.

0487:-8
0488:-12
0489:-12
0591:0

I'm not sure how this would be implemented yet, but emulating the commands we are using might be a good start, see- receipt-print-hq/escpos-tools#63 for that.