text,widget,opentype: change text.Face.Shape to return a clip.PathSpec

With this change, the Shape function returns a clip.PathSpec
instead of a clip.Outline op. It is then possible to create
a clip.Outline or clip.Stroke op to fill the text path or
draw its stroke.

Signed-off-by: Christophe Meessen <meessen@cppm.in2p3.fr>

text/lru_test.go

@@ -24,7 +24,7 @@ func TestLayoutLRU(t *testing.T) {
 func TestPathLRU(t *testing.T) {
 	c := new(pathCache)
 	put := func(i int) {
-		c.Put(pathKey{str: strconv.Itoa(i)}, clip.Op{})
+		c.Put(pathKey{str: strconv.Itoa(i)}, clip.PathSpec{})
 	}
 	get := func(i int) bool {
 		_, ok := c.Get(pathKey{str: strconv.Itoa(i)})