Add Quadratic Bezier Curve

[ajstarks]

Problem / Use Case

No support for Quadratic Bezier curves. My application uses these.

Proposed Solution

opts := &creator.BezierOptions{
    Color: creator.Blue,
    Width: 2.0,
}
segments := []creator.BezierSegment{
    {
        Start: creator.Point{X: 100, Y: 100},
        C1:    creator.Point{X: 150, Y: 200},
        End:   creator.Point{X: 250, Y: 100},
    },
}
err := page.DrawQuadBezierCurve(segments, opts)

Alternatives Considered

Attempting to derive Quad Bezier with the existing Cubic Bezier failed, Cubic curves cannot be represented exactly by a single quadratic curve.

Additional Context

[kolkov]

Done in #49. Here's what the API looks like:

segments := []creator.QuadBezierSegment{
    {
        Start:   creator.Point{X: 100, Y: 100},
        Control: creator.Point{X: 150, Y: 200},
        End:     creator.Point{X: 250, Y: 100},
    },
}
opts := &creator.BezierOptions{Color: creator.Blue, Width: 2.0}
err := page.DrawQuadBezierCurve(segments, opts)

Under the hood it converts to cubic via degree elevation (exact, not an approximation) since PDF only has cubic curves natively. There's also seg.ToCubic() if you need the conversion directly.

Multi-segment paths, fill, dash, opacity — all work. On main, going into v0.5.0.