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>