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>

op/clip/shapes.go

@@ -36,7 +36,10 @@ func (rr RRect) Op(ops *op.Ops) Op {
 	p.Begin(ops)
 	p.Move(rr.Rect.Min)
 	roundRect(&p, rr.Rect.Size(), rr.SE, rr.SW, rr.NW, rr.NE)
-	return p.Outline()
+
+	return Outline{
+		Path: p.End(),
+	}.Op()
 }
 
 // Add the rectangle clip.
@@ -49,7 +52,6 @@ type Border struct {
 	// Rect is the bounds of the border.
 	Rect  f32.Rectangle
 	Width float32
-	Style StrokeStyle
 	// The corner radii.
 	SE, SW, NW, NE float32
 }
@@ -58,11 +60,15 @@ type Border struct {
 func (b Border) Op(ops *op.Ops) Op {
 	var p Path
 	p.Begin(ops)
-
 	p.Move(b.Rect.Min)
 	roundRect(&p, b.Rect.Size(), b.SE, b.SW, b.NW, b.NE)
 
-	return p.Stroke(b.Width, b.Style)
+	return Stroke{
+		Path: p.End(),
+		Style: StrokeStyle{
+			Width: b.Width,
+		},
+	}.Op()
 }
 
 // Add the border clip.