feat: Use nearest-neighbor scaling in pi.Stretch

Because it is more accurate than the current algorithm and the performance is similar.

Author: elgopher

_examples/stretchtest/gamepad.png

@@ -0,0 +1,43 @@
+// Copyright 2025 Jacek Olszak
+// This code is licensed under MIT license (see LICENSE for details)
+
+package main
+
+import (
+	_ "embed"
+	"github.com/elgopher/pi"
+	"github.com/elgopher/pi/piebiten"
+	"github.com/elgopher/pi/pikey"
+	"github.com/elgopher/pi/piscope"
+)
+
+//go:embed "gamepad.png"
+var gamepadPNG []byte
+
+func main() {
+	pi.SetScreenSize(100, 60)
+	pi.Palette = pi.DecodePalette(gamepadPNG)
+	canvas := pi.DecodeCanvas(gamepadPNG)
+	spr := pi.SpriteFrom(canvas, 58, 26, 9, 9)
+	piscope.Start()
+	posx := 0
+	posy := 0
+	pi.Update = func() {
+		if pikey.Duration(pikey.Left) > 0 {
+			posx--
+		}
+		if pikey.Duration(pikey.Right) > 0 {
+			posx++
+		}
+		if pikey.Duration(pikey.Up) > 0 {
+			posy--
+		}
+		if pikey.Duration(pikey.Down) > 0 {
+			posy++
+		}
+	}
+	pi.Draw = func() {
+		pi.DrawSprite(spr, posx, posy)
+	}
+	piebiten.Run()
+}