text: remove String, Layout and add Glyph

In preparation for using Shaper with an io.Reader, rework the API to not refer
to strings. In particular, introduce Glyph for holding the rune in addition to
the advance. For fast traversing of the underlying text, add Len to Line with
the UTF8 length.

Layout is a useless wrapper around []Line; remove it while we're
here.

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

text/text.go

@@ -11,7 +11,9 @@ import (
 
 // A Line contains the measurements of a line of text.
 type Line struct {
-	Text String
+	Layout []Glyph
+	// Len is the length in UTF8 bytes of the line.
+	Len int
 	// Width is the width of the line.
 	Width fixed.Int26_6
 	// Ascent is the height above the baseline.
@@ -23,16 +25,9 @@ type Line struct {
 	Bounds fixed.Rectangle26_6
 }
 
-type String struct {
-	String string
-	// Advances contain the advance of each rune in String.
-	Advances []fixed.Int26_6
-}
-
-// A Layout contains the measurements of a body of text as
-// a list of Lines.
-type Layout struct {
-	Lines []Line
+type Glyph struct {
+	Rune    rune
+	Advance fixed.Int26_6
 }
 
 // LayoutOptions specify the constraints of a text layout.
@@ -59,8 +54,8 @@ type Font struct {
 
 // Face implements text layout and shaping for a particular font.
 type Face interface {
-	Layout(ppem fixed.Int26_6, str string, opts LayoutOptions) *Layout
-	Shape(ppem fixed.Int26_6, str String) op.CallOp
+	Layout(ppem fixed.Int26_6, str string, opts LayoutOptions) []Line
+	Shape(ppem fixed.Int26_6, str []Glyph) op.CallOp
 	Metrics(ppem fixed.Int26_6) font.Metrics
 }