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>

gpu/internal/rendertest/clip_test.go

@@ -91,6 +91,7 @@ func TestPaintArc(t *testing.T) {
 		p.Arc(f32.Pt(-10, -20), f32.Pt(10, -5), math.Pi)
 		p.Line(f32.Pt(0, -10))
 		p.Line(f32.Pt(-50, 0))
+		p.Close()
 		clip.Outline{
 			Path: p.End(),
 		}.Op().Add(o)
@@ -112,6 +113,7 @@ func TestPaintAbsolute(t *testing.T) {
 		p.MoveTo(f32.Pt(20, 20))
 		p.LineTo(f32.Pt(80, 20))
 		p.QuadTo(f32.Pt(80, 80), f32.Pt(20, 80))
+		p.Close()
 		clip.Outline{
 			Path: p.End(),
 		}.Op().Add(o)