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>

internal/rendertest/util_test.go

@@ -14,6 +14,7 @@ import (
 	"testing"
 
 	"gioui.org/app/headless"
+	"gioui.org/f32"
 	"gioui.org/op"
 	"gioui.org/op/paint"
 	"golang.org/x/image/colornames"
@@ -210,3 +211,7 @@ func newWindow(t testing.TB, width, height int) *headless.Window {
 	t.Cleanup(w.Release)
 	return w
 }
+
+func scale(sx, sy float32) op.TransformOp {
+	return op.Affine(f32.Affine2D{}.Scale(f32.Point{}, f32.Pt(sx, sy)))
+}