plot/vg: consider using golang.org/x/image/font/sfnt for parsing fonts

[sbinet]

right now, we are using "github.com/golang/freetype" and "github.com/golang/freetype/truetype". They work fine, but the license is a dual one (BSD-like or GPL2+) : https://github.com/golang/freetype/blob/master/LICENSE

We should perhaps consider use instead the new: https://godoc.org/golang.org/x/image/font/sfnt

(which is released under the same license than Go itself (BSD-3))

for this to happen, though, the following methods will need to be implemented for sfnt.Font:

• ~~sfnt.Font.Bounds~~
• ~~sfnt.Font.Index~~
• ~~sfnt.Font.Kern~~
• sfnt.Font.HMetric

[sbinet]

https://go-review.googlesource.com/c/37410/ added sfnt.Font.Kern.

[sbinet]

https://go-review.googlesource.com/39670 added sfnt.Font.Bounds.

[sbinet]

https://go-review.googlesource.com/#/c/67330/ is adding metrics to font.

[sbinet]

freetype/truetype.Font.Index is spelled sfnt.GlyphIndex.

[sbinet]

this has been done at some point.