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/shaper.go

@@ -0,0 +1,109 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package text
+
+import (
+	"gioui.org/op/paint"
+	"gioui.org/unit"
+	"golang.org/x/image/math/fixed"
+)
+
+// Shaper implements layout and shaping of text and a cache of
+// computed results.
+//
+// Specify the default and fallback font by calling Register with the
+// empty Font.
+type Shaper struct {
+	faces map[Font]*face
+}
+
+type face struct {
+	face        Face
+	layoutCache layoutCache
+	pathCache   pathCache
+}
+
+func (s *Shaper) Register(font Font, tf Face) {
+	if s.faces == nil {
+		s.faces = make(map[Font]*face)
+	}
+	// Treat all font sizes equally.
+	font.Size = unit.Value{}
+	if font.Weight == 0 {
+		font.Weight = Normal
+	}
+	s.faces[font] = &face{
+		face: tf,
+	}
+}
+
+func (s *Shaper) Layout(c unit.Converter, font Font, str string, opts LayoutOptions) *Layout {
+	tf := s.faceForFont(font)
+	return tf.layout(fixed.I(c.Px(font.Size)), str, opts)
+}
+
+func (s *Shaper) Shape(c unit.Converter, font Font, str String) paint.ClipOp {
+	tf := s.faceForFont(font)
+	return tf.shape(fixed.I(c.Px(font.Size)), str)
+}
+
+func (s *Shaper) faceForStyle(font Font) *face {
+	tf := s.faces[font]
+	if tf == nil {
+		font := font
+		font.Weight = Normal
+		tf = s.faces[font]
+	}
+	if tf == nil {
+		font := font
+		font.Style = Regular
+		tf = s.faces[font]
+	}
+	if tf == nil {
+		font := font
+		font.Style = Regular
+		font.Weight = Normal
+		tf = s.faces[font]
+	}
+	return tf
+}
+
+func (s *Shaper) faceForFont(font Font) *face {
+	font.Size = unit.Value{}
+	tf := s.faceForStyle(font)
+	if tf == nil {
+		font.Typeface = ""
+		tf = s.faceForStyle(font)
+	}
+	if tf == nil {
+		panic("no default typeface defined")
+	}
+	return tf
+}
+
+func (t *face) layout(ppem fixed.Int26_6, str string, opts LayoutOptions) *Layout {
+	lk := layoutKey{
+		ppem: ppem,
+		str:  str,
+		opts: opts,
+	}
+	if l, ok := t.layoutCache.Get(lk); ok {
+		return l
+	}
+	l := t.face.Layout(ppem, str, opts)
+	t.layoutCache.Put(lk, l)
+	return l
+}
+
+func (t *face) shape(ppem fixed.Int26_6, str String) paint.ClipOp {
+	pk := pathKey{
+		ppem: ppem,
+		str:  str.String,
+	}
+	if clip, ok := t.pathCache.Get(pk); ok {
+		return clip
+	}
+	clip := t.face.Shape(ppem, str)
+	t.pathCache.Put(pk, clip)
+	return clip
+}