internal/stroke,op/clip: don't import op/clip from internal/stroke

To avoid an import cycle in a future change, internal/stroke can no
longer import op/clip. Move required op/clip functionality to
internal/stroke and duplicate the remaining types.

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

gpu/compute.go

@@ -708,7 +708,13 @@ func encodePath(p *pathOp) encoder {
 	verts := p.pathVerts
 	if p.stroke.Width > 0 && !supportsStroke(p) {
 		quads := decodeToStrokeQuads(verts)
-		quads = quads.Stroke(p.stroke, p.dashes)
+		ss := stroke.StrokeStyle{
+			Width: p.stroke.Width,
+			Miter: p.stroke.Miter,
+			Cap:   stroke.StrokeCap(p.stroke.Cap),
+			Join:  stroke.StrokeJoin(p.stroke.Join),
+		}
+		quads = quads.Stroke(ss, p.dashes)
 		for _, quad := range quads {
 			q := quad.Quad
 			enc.quad(q.From, q.Ctrl, q.To)