Add stroke functionality to glyphs

[tdewolff]

This adds the ability to set stroke width for font drawing. Drawing a black stroked glyph and then a white normal glyph will give the font a black stroke.

The only problem is that the rasterizer doesn't allow closing paths. If the start and end point are at the same position, the strokes will overlap and appear thicker (alpha channel overlaps). If the start and end points are at an angle, using ButtCapping will reveal that they do not properly connect. The rasterizer must support path closing for this.

Secondly, this should be added to github.com/golang/freetype/freetype.go too?

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[tdewolff]

I signed it!

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CLAs look good, thanks!

[tdewolff]

Closed in favor of #49