Add support for stroke in canvas

[LunaTheFoxgirl]

From what I can tell there's currently no stroke function in canvas, which makes it difficult to render line drawings of, for example, waveforms.

It would be nice if such functionality was present.

[p0nce]

Former discussion cerjones/dg2d#2 (comment)

[p0nce]

The way I see it would be a multi-month effort to do another rasterizer with

• 16-bit depth L16 output (since we can't remove all L16 drawing function else)
• Porter Duff operators
• Stroking

Unfortunately this dg2d fork is only well understood by its author, also dg2d is pretty quick by virtue of not having porter-duff or L16 or stroking, somehow (though stroking is just more functionality)

[p0nce]

Finally closed with #860