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/material/loader.go

@@ -48,9 +48,7 @@ func (l LoaderStyle) Layout(gtx layout.Context) layout.Dimensions {
 		Color: l.Color,
 	}.Add(gtx.Ops)
 	op.Offset(f32.Pt(-float32(radius), -float32(radius))).Add(gtx.Ops)
-	paint.PaintOp{
-		Rect: f32.Rectangle{Max: layout.FPt(sz)},
-	}.Add(gtx.Ops)
+	paint.PaintOp{}.Add(gtx.Ops)
 	op.InvalidateOp{}.Add(gtx.Ops)
 	return layout.Dimensions{
 		Size: sz,