op/paint: remove support for PaintOp.Rect

PaintOp.Rect is the wrong abstraction; it implies a clip operation
better handled by package clip, and not all paints need it (colors).
Furthermore, it's awkward to specify a PaintOp that fills up the
current clip area, regardless of its size.

Redefine PathOp to mean "fill current clip area".

API change. Replace uses of PaintOp.Rect with a TransformOp applied
before the PaintOp.

Leave a TODO for the PathOp infinity area.

Fixes gio#167

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

widget/border.go

@@ -33,11 +33,8 @@ func (b Border) Layout(gtx layout.Context, w layout.Widget) layout.Dimensions {
 		NE: rr, NW: rr, SE: rr, SW: rr,
 		Width: float32(width),
 	}.Add(gtx.Ops)
-	dr := f32.Rectangle{
-		Max: layout.FPt(sz),
-	}
 	paint.ColorOp{Color: b.Color}.Add(gtx.Ops)
-	paint.PaintOp{Rect: dr}.Add(gtx.Ops)
+	paint.PaintOp{}.Add(gtx.Ops)
 	st.Pop()
 	return dims
 }