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>

widget/material/loader.go

@@ -67,15 +67,17 @@ func clipLoader(ops *op.Ops, startAngle, endAngle, radius float64) {
 		pen    = f32.Pt(float32(vx), float32(vy)).Mul(float32(radius))
 		center = f32.Pt(0, 0).Sub(pen)
 
-		style = clip.StrokeStyle{
-			Cap: clip.FlatCap,
-		}
-
 		p clip.Path
 	)
 
 	p.Begin(ops)
 	p.Move(pen)
 	p.Arc(center, center, delta)
-	p.Stroke(width, style).Add(ops)
+	clip.Stroke{
+		Path: p.End(),
+		Style: clip.StrokeStyle{
+			Width: width,
+			Cap:   clip.FlatCap,
+		},
+	}.Op().Add(ops)
 }