op/clip: don't panic on zero-sized Ellipse

When the rectangle used in an Ellipse has no width
or height, the path would panic with "path not closed".
Use an empty rectangle path in that case instead.

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

op/clip/shapes.go

@@ -158,10 +158,14 @@ func (e Ellipse) Push(ops *op.Ops) Stack {
 
 // path constructs a path for the ellipse.
 func (e Ellipse) path(o *op.Ops) PathSpec {
+	bounds := f32.Rectangle(e)
+	if bounds.Dx() == 0 || bounds.Dy() == 0 {
+		return PathSpec{shape: ops.Rect}
+	}
+
 	var p Path
 	p.Begin(o)
 
-	bounds := f32.Rectangle(e)
 	center := bounds.Max.Add(bounds.Min).Mul(.5)
 	diam := bounds.Dx()
 	r := diam * .5

op/clip/shapes_test.go

@@ -0,0 +1,18 @@
+package clip_test
+
+import (
+	"image/color"
+	"testing"
+
+	"gioui.org/f32"
+	"gioui.org/op"
+	"gioui.org/op/clip"
+	"gioui.org/op/paint"
+)
+
+func TestZeroEllipse(t *testing.T) {
+	p := f32.Pt(1.0, 2.0)
+	e := clip.Ellipse{Min: p, Max: p}
+	ops := new(op.Ops)
+	paint.FillShape(ops, color.NRGBA{R: 255, A: 255}, e.Op(ops))
+}