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/switch.go

@@ -62,7 +62,7 @@ func (s SwitchStyle) Layout(gtx layout.Context) layout.Dimensions {
 		NE:   trackCorner, NW: trackCorner, SE: trackCorner, SW: trackCorner,
 	}.Add(gtx.Ops)
 	paint.ColorOp{Color: trackColor}.Add(gtx.Ops)
-	paint.PaintOp{Rect: trackRect}.Add(gtx.Ops)
+	paint.PaintOp{}.Add(gtx.Ops)
 	stack.Pop()
 
 	// Draw thumb ink.
@@ -133,5 +133,5 @@ func drawDisc(ops *op.Ops, sz float32, col color.RGBA) {
 		NE:   rr, NW: rr, SE: rr, SW: rr,
 	}.Add(ops)
 	paint.ColorOp{Color: col}.Add(ops)
-	paint.PaintOp{Rect: r}.Add(ops)
+	paint.PaintOp{}.Add(ops)
 }