func AddEllipse(cx, cy, rx, ry, rot float64, p Adder) {
	rotRads := rot * math.Pi / 180
	px, py := Identity.
		Translate(cx, cy).Rotate(rotRads).Translate(-cx, -cy).Transform(cx+rx, cy)
	points := []float64{rx, ry, rot, 1.0, 0.0, px, py}
	p.Start(ToFixedP(px, py))
	AddArc(points, cx, cy, px, py, p)
	p.Stop(true)
}
AddEllipse adds an elipse with center at cx,cy, with the indicated
x and y radius, (rx, ry), rotated around the center by rot degrees.
package rasterx
import (
	"math"
	"golang.org/x/image/math/fixed"
)
const MaxDx float64 = math.Pi / 8
func ToFixedP(x, y float64) (p fixed.Point26_6) {
	p.X = fixed.Int26_6(x * 64)
	p.Y = fixed.Int26_6(y * 64)
	return
}
func AddCircle(cx, cy, r float64, p Adder) {
	AddEllipse(cx, cy, r, r, 0, p)
}