Support kerningRTL from .glyphs files

[rsheeter]

#330 (review)

[anthrotype]

So in glyphs 3, there are two kerning dictionary, kerning_ltr and kerning_rtl (whereas in UFO and glyphs < 3 there is only one kerning dict), and the user decides what pair goes where while they are kerning by toggling some button; in fontmake (glyphsLib actually) we merge the two dicts into one, and then ufo2ft kern writer spits them based on the glyph unicode properties.

fontc currently only parses the kerning_ltr and completely ignores the other kerning_rtl dictionary.
The problem, as usual, is not as simple as just merging two dictionaries... because we also need to swap left/right groups for all glyphs that are used in kerningRTL.

For reference, see
googlefonts/glyphsLib#865

[simoncozens]

Note also that glyphsLib's implementation is incomplete (googlefonts/glyphsLib#1039).

[anthrotype]

fixed by #1570