[why]
We 'manually' patch the font file after `fontforge` created it. This can
go wrong, for example we fail to create a correct new checksum.
Or we fail to patch the correct font property.
[how]
Also build `showttf` from the `fontforge` package and use it to extract
some font properties:
* Check the example patched font file checksum
* Compare the `lowestRecPPEM` of source to patched font file
[note]
`fontforge` set `lowestRecPPEM` always to 8 in generated fonts.
Hack has a value of 6.
Signed-off-by: Fini Jastrow <ulf.fini.jastrow@desy.de>