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/button.go

@@ -287,5 +287,5 @@ func drawInk(gtx layout.Context, c widget.Press) {
 		}},
 		NE: rr, NW: rr, SE: rr, SW: rr,
 	}.Add(gtx.Ops)
-	paint.PaintOp{Rect: f32.Rectangle{Max: f32.Point{X: float32(size), Y: float32(size)}}}.Add(gtx.Ops)
+	paint.PaintOp{}.Add(gtx.Ops)
 }