op/paint: add nearest neighbor scaling

This adds support for nearest neighbor filtering,
which can be useful in quite a few scenarios.

  img := paint.NewImageOp(m)
  img.Filter = paint.FilterNearest
  img.Add(gtx.Ops)

Fixes: https://todo.sr.ht/~eliasnaur/gio/414
Signed-off-by: Egon Elbre <egonelbre@gmail.com>

gpu/internal/rendertest/render_test.go

@@ -359,6 +359,73 @@ func TestImageRGBA(t *testing.T) {
 	})
 }
 
+func TestImageRGBA_ScaleLinear(t *testing.T) {
+	run(t, func(o *op.Ops) {
+		w := newWindow(t, 128, 128)
+		defer clip.Rect{Max: image.Pt(128, 128)}.Push(o).Pop()
+		op.Affine(f32.Affine2D{}.Scale(f32.Point{}, f32.Pt(64, 64))).Add(o)
+
+		im := image.NewRGBA(image.Rect(0, 0, 2, 2))
+		im.Set(0, 0, colornames.Red)
+		im.Set(1, 0, colornames.Green)
+		im.Set(0, 1, colornames.White)
+		im.Set(1, 1, colornames.Black)
+
+		op := paint.NewImageOp(im)
+		op.Filter = paint.FilterLinear
+		op.Add(o)
+
+		paint.PaintOp{}.Add(o)
+
+		if err := w.Frame(o); err != nil {
+			t.Error(err)
+		}
+	}, func(r result) {
+		r.expect(0, 0, colornames.Red)
+		r.expect(8, 8, colornames.Red)
+
+		// TODO: this currently seems to do srgb scaling
+		// instead of linear rgb scaling,
+		r.expect(64-4, 0, color.RGBA{R: 197, G: 87, B: 0, A: 255})
+		r.expect(64+4, 0, color.RGBA{R: 175, G: 98, B: 0, A: 255})
+
+		r.expect(127, 0, colornames.Green)
+		r.expect(127-8, 8, colornames.Green)
+	})
+}
+
+func TestImageRGBA_ScaleNearest(t *testing.T) {
+	run(t, func(o *op.Ops) {
+		w := newWindow(t, 128, 128)
+		op.Affine(f32.Affine2D{}.Scale(f32.Point{}, f32.Pt(64, 64))).Add(o)
+
+		im := image.NewRGBA(image.Rect(0, 0, 2, 2))
+		im.Set(0, 0, colornames.Red)
+		im.Set(1, 0, colornames.Green)
+		im.Set(0, 1, colornames.White)
+		im.Set(1, 1, colornames.Black)
+
+		op := paint.NewImageOp(im)
+		op.Filter = paint.FilterNearest
+		op.Add(o)
+
+		paint.PaintOp{}.Add(o)
+
+		if err := w.Frame(o); err != nil {
+			t.Error(err)
+		}
+	}, func(r result) {
+		r.expect(0, 0, colornames.Red)
+		r.expect(8, 8, colornames.Red)
+
+		r.expect(64-4, 0, colornames.Red)
+		r.expect(64+4, 0, colornames.Green)
+
+		r.expect(127, 0, colornames.Green)
+		r.expect(127-8, 8, colornames.Green)
+	})
+}
+
 func TestGapsInPath(t *testing.T) {
 	ops := new(op.Ops)
 	var p clip.Path