all: introduce Outline and Stroke builders

This CL introduces 2 new path builders:
- Outline which takes a PathSpec to be outlined
- Stroke which takes a PathSpec and a stroke style, to stroke a path.

typically, code like this:

  var p clip.Path
  ...
  p.Outline().Add(o)

should be replaced with:

  var p clip.Path
  ...
  clip.Outline{Path: p.End()}.Op().Add(o)

similarly, stroking should be modified from:

  var p clip.Path
  ...
  p.Stroke(width, clip.StrokeStyle{...}).Add(o)

to:

  var p clip.Path
  ...
  clip.Stroke{Path: p.End(), Style: clip.StrokeStyle{Width:...}}.Op().Add(o)

here are tentative 'rf' scripts (see rsc.io/rf for more details):

  ```
  ex {
  	import "gioui.org/op";
  	import "gioui.org/op/clip";

  	var p clip.Path;
  	var o *op.Ops;

  	p.Outline().Add(o) -> clip.Outline{Path:p.End()}.Op().Add(o);
  }

  ex {
  	import "gioui.org/op";
  	import "gioui.org/op/clip";

  	var o *op.Ops;
  	var p clip.Path;
  	var sty clip.StrokeStyle;
  	var width float32;

  	p.Stroke(width, sty).Add(o) ->   \
	    clip.Stroke{                 \
		Path:p.End(),            \
		Style: clip.StrokeStyle{ \
		    Width: width,        \
	    }}.Op().Add(o);
  }
  ```

Signed-off-by: Sebastien Binet <s@sbinet.org>

internal/rendertest/render_test.go

@@ -71,7 +71,10 @@ func TestRepeatedPaintsZ(t *testing.T) {
 		builder.Line(f32.Pt(0, 10))
 		builder.Line(f32.Pt(-10, 0))
 		builder.Line(f32.Pt(0, -10))
-		builder.Outline().Add(o)
+		p := builder.End()
+		clip.Outline{
+			Path: p,
+		}.Op().Add(o)
 		paint.Fill(o, red)
 	}, func(r result) {
 		r.expect(5, 5, colornames.Red)
@@ -109,7 +112,8 @@ func constSqPath() op.CallOp {
 	builder.Line(f32.Pt(0, 10))
 	builder.Line(f32.Pt(-10, 0))
 	builder.Line(f32.Pt(0, -10))
-	builder.Outline().Add(innerOps)
+	p := builder.End()
+	clip.Outline{Path: p}.Op().Add(innerOps)
 	return m.Stop()
 }