op/clip: don't accept open Paths for Outline

Outline represents a clipping operations that clips all drawing outside
a closed path. Before this change, paths not closed we're patched up by
adding an implicit line from the endpoint to the beginning.

These fixups are inefficient for a rare case, but acceptable because the
old renderer post-processes all paths anyway. However, the new compute
renderer don't need post-processing in most cases, making fixups too
expensive.

Given that clipping to an open path is fundamentally undefined and that
implicit fixup with a closing line segment is merely a way to force the
clip to be well-defined, this change adds a panic to Outline for Paths
that are not closed.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

op/clip/clip_test.go

@@ -0,0 +1,22 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package clip
+
+import (
+	"testing"
+
+	"gioui.org/f32"
+	"gioui.org/op"
+)
+
+func TestOpenPathOutlinePanic(t *testing.T) {
+	defer func() {
+		if err := recover(); err == nil {
+			t.Error("Outline of an open path didn't panic")
+		}
+	}()
+	var p Path
+	p.Begin(new(op.Ops))
+	p.Line(f32.Pt(10, 10))
+	Outline{Path: p.End()}.Op()
+}