Partial transcoding of corner components

If I run glyphs2ufo on a font that includes corner components, say [Inconsolata](https://github.com/googlefonts/Inconsolata/blob/118fac929245c210192be8b1be695a6a65340e97/sources/Inconsolata.glyphs) (that's a link to current master, not the VF branch I'm currently working on), then corner components are not translated properly to UFO.

Here's the relevant extract from the glyphs file (glyph /h, this is line 20440 of the linked file):

```
hints = (
{
origin = "{0, 17}";
type = Corner;
name = _corner.rightSpur;
}
);
```

And here's the relevant excerpt from the h.glif file:

```xml

com.schriftgestaltung.hints


horizontal

options
0
origin

0
17

stem
-2
type
Corner



```

The `name` field is gone, which is of course critical, and there are spurious additional fields for `horizontal`, `options`, and `stem`.

It's not too hard to figure out what's going on here. Very likely Glyphs has chosen a hacky way to represent corner components, overloading its "hint" key, and the relevant [glyphslib code](https://github.com/googlefonts/glyphsLib/blob/master/Lib/glyphsLib/builder/hints.py) is doing its best to transcode a hint. The code needs to be adapted to switch on the `type` field and handle it differently in the case of corners.