Respect 'Original kern groups'

[cmyr]

See #1728.

This patch has some pros and cons:

cons:

• it is replicating behaviour that we know to be buggy (Support Glyphs3 RTL kerning glyphsLib#778 (comment), https://github.com/googlefonts/glyphsLib/blob/f90e4060b/Lib/glyphsLib/builder/groups.py#L109-L112)
• it adds a bunch of mysterious logic that doesn't really do anything useful
• the actual behaviour we're 'fixing' here is ultimately probably an error in one source file (an LTR kern being added to RTL by accident)
• I hate it

pros

• green number go up