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/opconst/ops.go

@@ -31,6 +31,8 @@ const (
 	TypeClip
 	TypeProfile
 	TypeCursor
+	TypePath
+	TypeStroke
 )
 
 const (
@@ -54,9 +56,11 @@ const (
 	TypePushLen            = 1
 	TypePopLen             = 1
 	TypeAuxLen             = 1
-	TypeClipLen            = 1 + 4*4 + 4 + 2 + 4
+	TypeClipLen            = 1 + 4*4 + 1
 	TypeProfileLen         = 1
 	TypeCursorLen          = 1 + 1
+	TypePathLen            = 1 + 4
+	TypeStrokeLen          = 1 + 4 + 4 + 1 + 1
 )
 
 func (t OpType) Size() int {
@@ -84,6 +88,8 @@ func (t OpType) Size() int {
 		TypeClipLen,
 		TypeProfileLen,
 		TypeCursorLen,
+		TypePathLen,
+		TypeStrokeLen,
 	}[t-firstOpIndex]
 }