text: replace Family with Shaper, add Font, Face

There is now a single shaping implementation, Shaper, for all fonts, replacing
Family that only covered a single typeface.

A typeface is identified by a name, where the empty string denotes the
default typeface.

Font is introduced to specify a particular font from the typeface, style,
weight and size.

Face is changed to an interface for a particular layout and shaping method.
The text/shape package is renamed to text/opentype and contains a Face
implementation based on golang.org/x/image/font/sfnt.

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

text/text.go

@@ -0,0 +1,141 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package text
+
+import (
+	"fmt"
+	"image"
+
+	"gioui.org/layout"
+	"gioui.org/op/paint"
+	"gioui.org/unit"
+	"golang.org/x/image/math/fixed"
+)
+
+// A Line contains the measurements of a line of text.
+type Line struct {
+	Text String
+	// Width is the width of the line.
+	Width fixed.Int26_6
+	// Ascent is the height above the baseline.
+	Ascent fixed.Int26_6
+	// Descent is the height below the baseline, including
+	// the line gap.
+	Descent fixed.Int26_6
+	// Bounds is the visible bounds of the line.
+	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
+}
+
+// LayoutOptions specify the constraints of a text layout.
+type LayoutOptions struct {
+	// MaxWidth set the maximum width of the layout.
+	MaxWidth int
+	// SingleLine specify that line breaks are ignored.
+	SingleLine bool
+}
+
+// Style is the font style.
+type Style int
+
+// Weight is a font weight, in CSS units.
+type Weight int
+
+// Font specify a particular typeface, style and size.
+type Font struct {
+	// Typeface identifies a particular typeface design. The empty
+	// string denotes the default typeface.
+	Typeface string
+	Size     unit.Value
+	Style    Style
+	// Weight is the text weight. If zero, Normal is used instead.
+	Weight Weight
+}
+
+// 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) paint.ClipOp
+}
+
+type Alignment uint8
+
+const (
+	Start Alignment = iota
+	End
+	Middle
+)
+
+const (
+	Regular Style = iota
+	Italic
+)
+
+const (
+	Normal Weight = 400
+	Bold   Weight = 700
+)
+
+func linesDimens(lines []Line) layout.Dimensions {
+	var width fixed.Int26_6
+	var h int
+	var baseline int
+	if len(lines) > 0 {
+		baseline = lines[0].Ascent.Ceil()
+		var prevDesc fixed.Int26_6
+		for _, l := range lines {
+			h += (prevDesc + l.Ascent).Ceil()
+			prevDesc = l.Descent
+			if l.Width > width {
+				width = l.Width
+			}
+		}
+		h += lines[len(lines)-1].Descent.Ceil()
+	}
+	w := width.Ceil()
+	return layout.Dimensions{
+		Size: image.Point{
+			X: w,
+			Y: h,
+		},
+		Baseline: baseline,
+	}
+}
+
+func align(align Alignment, width fixed.Int26_6, maxWidth int) fixed.Int26_6 {
+	mw := fixed.I(maxWidth)
+	switch align {
+	case Middle:
+		return fixed.I(((mw - width) / 2).Floor())
+	case End:
+		return fixed.I((mw - width).Floor())
+	case Start:
+		return 0
+	default:
+		panic(fmt.Errorf("unknown alignment %v", align))
+	}
+}
+
+func (a Alignment) String() string {
+	switch a {
+	case Start:
+		return "Start"
+	case End:
+		return "End"
+	case Middle:
+		return "Middle"
+	default:
+		panic("unreachable")
+	}
+}