ui: split OpImage into OpImage and OpDraw

In preparation for an OpColor (and future OpGradient and similar).

Label and Editor no longer take an explicit source image. They
draw with the current image.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

ui/widget/image.go

@@ -27,6 +27,7 @@ func (im Image) Layout(ops *ui.Ops, cs layout.Constraints) layout.Dimens {
 	dr := f32.Rectangle{
 		Max: f32.Point{X: float32(d.X), Y: float32(d.Y)},
 	}
-	draw.OpImage{Rect: dr, Src: im.Src, SrcRect: im.Rect}.Add(ops)
+	draw.OpImage{Img: im.Src, Rect: im.Rect}.Add(ops)
+	draw.OpDraw{Rect: dr}.Add(ops)
 	return layout.Dimens{Size: d, Baseline: d.Y}
 }