op/clip: don't panic when Path is used without an initial MoveTo

A Path initialized with Begin should be ready to use with its pen at (0,
0). Make it so.

Updates gio#311

Signed-off-by: Pierre Curto <pierre.curto@gmail.com>

op/clip/clip.go

@@ -124,10 +124,12 @@ func (p *Path) Pos() f32.Point { return p.pen }
 
 // Begin the path, storing the path data and final Op into ops.
 func (p *Path) Begin(o *op.Ops) {
+	*p = Path{
+		ops:     &o.Internal,
+		macro:   op.Record(o),
+		contour: 1,
+	}
 	p.hash.SetSeed(pathSeed)
-	p.ops = &o.Internal
-	p.macro = op.Record(o)
-	// Write the TypeAux opcode
 	data := ops.Write(p.ops, ops.TypeAuxLen)
 	data[0] = byte(ops.TypeAux)
 }

op/clip/clip_test.go

@@ -1,12 +1,16 @@
 // SPDX-License-Identifier: Unlicense OR MIT
 
-package clip
+package clip_test
 
 import (
+	"image/color"
 	"testing"
 
 	"gioui.org/f32"
+	"gioui.org/gpu/headless"
 	"gioui.org/op"
+	"gioui.org/op/clip"
+	"gioui.org/op/paint"
 )
 
 func TestOpenPathOutlinePanic(t *testing.T) {
@@ -15,10 +19,27 @@ func TestOpenPathOutlinePanic(t *testing.T) {
 			t.Error("Outline of an open path didn't panic")
 		}
 	}()
-	var p Path
+	var p clip.Path
 	p.Begin(new(op.Ops))
 	p.Line(f32.Pt(10, 10))
-	Outline{Path: p.End()}.Op()
+	clip.Outline{Path: p.End()}.Op()
+}
+
+func TestPathBegin(t *testing.T) {
+	ops := new(op.Ops)
+	var p clip.Path
+	p.Begin(ops)
+	p.LineTo(f32.Point{10, 10})
+	p.Close()
+	stack := clip.Outline{Path: p.End()}.Op().Push(ops)
+	paint.Fill(ops, color.NRGBA{A: 255})
+	stack.Pop()
+	w := newWindow(t, 100, 100)
+	if w == nil {
+		return
+	}
+	// The following should not panic.
+	_ = w.Frame(ops)
 }
 
 func TestTransformChecks(t *testing.T) {
@@ -28,7 +49,15 @@ func TestTransformChecks(t *testing.T) {
 		}
 	}()
 	var ops op.Ops
-	st := Op{}.Push(&ops)
+	st := clip.Op{}.Push(&ops)
 	op.Record(&ops)
 	st.Pop()
 }
+
+func newWindow(t testing.TB, width, height int) *headless.Window {
+	w, err := headless.NewWindow(width, height)
+	if err != nil {
+		t.Skipf("failed to create headless window, skipping: %v", err)
+	}
+	return w
+}