font/plan9font: derive the example's ascent from the metrics.

The Face.Metrics method is relatively new.

Also pick up the unicode.7x13.font fix from the upstream plan9port:
https://plan9port-review.googlesource.com/#/c/1490/

Change-Id: Ia6e0feec4ea3c9a35e19d38aa634ca9eb6b2ffa0
Reviewed-on: https://go-review.googlesource.com/21699
Reviewed-by: Rob Pike <r@golang.org>

font/plan9font/example_test.go

@@ -30,8 +30,7 @@ func ExampleParseFont() {
 	if err != nil {
 		log.Fatal(err)
 	}
-	// TODO: derive the ascent from the face's metrics.
-	const ascent = 11
+	ascent := face.Metrics().Ascent.Ceil()
 
 	dst := image.NewRGBA(image.Rect(0, 0, 4*7, 13))
 	draw.Draw(dst, dst.Bounds(), image.Black, image.Point{}, draw.Src)

font/testdata/fixed/unicode.7x13.font

@@ -1,4 +1,4 @@
-13 10
+13 11
 0x0000 0x001F 7x13.2400
 0x0000 0x00FF 7x13.0000
 0x0100 0x01FF 7x13.0100