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/icon.go

@@ -7,7 +7,6 @@ import (
 	"image/color"
 	"image/draw"
 
-	"gioui.org/f32"
 	"gioui.org/layout"
 	"gioui.org/op/paint"
 	"gioui.org/unit"
@@ -35,11 +34,7 @@ func NewIcon(data []byte) (*Icon, error) {
 func (ic *Icon) Layout(gtx layout.Context, sz unit.Value) layout.Dimensions {
 	ico := ic.image(gtx.Px(sz))
 	ico.Add(gtx.Ops)
-	paint.PaintOp{
-		Rect: f32.Rectangle{
-			Max: layout.FPt(ico.Size()),
-		},
-	}.Add(gtx.Ops)
+	paint.PaintOp{}.Add(gtx.Ops)
 	return layout.Dimensions{
 		Size: ico.Size(),
 	}