Unify the implementation of builder for path and that for stroke and fill

[yutannihilation]

Fix #69

The current implementation flatten curves in a different way for path:

• In the cases of stroke and fill, flattening is done in tessellator via options provided by lyon (StrokeOptions and FillOptions).
• In the case of path, path doesn't use tessellator. Instead, it constructs CubicBezierSegment or QuadraticBezierSegment on the fly and flatten it.

It seems the latter way produces different results than the first one (probably the order of scaling operations might matter...?). This pull request changes this by making flattening happen in the Builder. This requires a bit tricky setup.

• stroke and fill: do not flatten in PathBuilder because the tesselator flattens later
• path: flatten in PathBuilder

---

    p <- string2path("s", "Arial", tolerance = 3e-3)
    f <- string2fill("s", "Arial", tolerance = 3e-3)
    
    plot(NULL, xlim = c(0, 0.45), ylim = c(0, 0.45))
    for (tri in split(f, f$triangle_id)) {
      polygon(tri$x, tri$y, col = "#ff3344", border = "transparent")
    }
    lines(p$x, p$y, lwd = 10)

Before:

After: