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>

internal/ops/ops.go

@@ -142,7 +142,7 @@ const (
 	TypePushOpacityLen      = 1 + 4
 	TypePopOpacityLen       = 1
 	TypeRedrawLen           = 1 + 8
-	TypeImageLen            = 1
+	TypeImageLen            = 1 + 1
 	TypePaintLen            = 1
 	TypeColorLen            = 1 + 4
 	TypeLinearGradientLen   = 1 + 8*2 + 4*2