Add stroke functionality to the 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). An upcoming PR will solve this.

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