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/progressbar.go

@@ -32,9 +32,6 @@ func (p ProgressBarStyle) Layout(gtx layout.Context) layout.Dimensions {
 		rr := float32(gtx.Px(unit.Dp(2)))
 
 		d := image.Point{X: int(width), Y: gtx.Px(maxHeight)}
-		dr := f32.Rectangle{
-			Max: f32.Point{X: float32(d.X), Y: float32(d.Y)},
-		}
 
 		clip.RRect{
 			Rect: f32.Rectangle{Max: f32.Point{X: width, Y: float32(gtx.Px(maxHeight))}},
@@ -42,7 +39,7 @@ func (p ProgressBarStyle) Layout(gtx layout.Context) layout.Dimensions {
 		}.Add(gtx.Ops)
 
 		paint.ColorOp{Color: color}.Add(gtx.Ops)
-		paint.PaintOp{Rect: dr}.Add(gtx.Ops)
+		paint.PaintOp{}.Add(gtx.Ops)
 
 		return layout.Dimensions{Size: d}
 	}