font/opentype: [API] support font collection loading

This commit adds back support for loading font collections, which we
lost when switching to the harfbuzz-based shaper last January. In
addition, this commit takes advantage of our new font loading library's
metadata facilities to automatically construct text.FontFaces for all
fonts within a collection. This is significantly more ergonomic for
users, and can be used to load single fonts with automatic metadata
detection as well.

I've exposed a opentype.Face.Font() method that can be used to get the
font metadata for a given face as well, though you have to type assert to
see it:

var myFace text.Face
if asOpentype, ok := myFace.(opentype.Face); ok {
    myFont := asOpentype.Font()
}

The one problem with this approach is that the font variant field always
be automatically populated. Mono font detection is supported, but
other variants like SmallCaps are more complicated and may need to be
expressed differently in the future (smallcaps is a feature that any font
file can have, not necessarily a separate font file). See this [0] upstream
issue for details.

Additionally, in order to avoid import cycles, I've moved the declarations
of font attributes to package font. You can fix your code automatically to
refer to the new definitions by running the following:

    gofmt -w -r 'text.FontFace -> font.FontFace' .
    gofmt -w -r 'text.Variant -> font.Variant' .
    gofmt -w -r 'text.Style -> font.Style' .
    gofmt -w -r 'text.Typeface -> font.Typeface' .
    gofmt -w -r 'text.Font -> font.Font' .
    gofmt -w -r 'text.Regular -> font.Regular' .
    gofmt -w -r 'text.Italic -> font.Italic' .
    gofmt -w -r 'text.Thin -> font.Thin' .
    gofmt -w -r 'text.ExtraLight -> font.ExtraLight' .
    gofmt -w -r 'text.Light -> font.Light' .
    gofmt -w -r 'text.Normal -> font.Normal' .
    gofmt -w -r 'text.Medium -> font.Medium' .
    gofmt -w -r 'text.SemiBold -> font.SemiBold' .
    gofmt -w -r 'text.Bold -> font.Bold' .
    gofmt -w -r 'text.ExtraBold -> font.ExtraBold' .
    gofmt -w -r 'text.Black -> font.Black' .
    gofmt -w -r 'text.Hairline -> font.Thin' .
    gofmt -w -r 'text.UltraLight -> font.ExtraLight' .
    gofmt -w -r 'text.DemiBold -> font.SemiBold' .
    gofmt -w -r 'text.UltraBold -> font.ExtraBold' .
    gofmt -w -r 'text.Heavy -> font.Black' .
    gofmt -w -r 'text.ExtraBlack -> font.Black+50' .
    gofmt -w -r 'text.UltraBlack -> font.ExtraBlack' .

Make sure each affected file imports gioui.org/font.

[0] https://github.com/go-text/typesetting/issues/57

Signed-off-by: Chris Waldon <christopher.waldon.dev@gmail.com>

widget/material/button.go

@@ -7,6 +7,7 @@ import (
 	"image/color"
 	"math"
 
+	"gioui.org/font"
 	"gioui.org/internal/f32color"
 	"gioui.org/io/semantic"
 	"gioui.org/layout"
@@ -22,7 +23,7 @@ type ButtonStyle struct {
 	Text string
 	// Color is the text color.
 	Color        color.NRGBA
-	Font         text.Font
+	Font         font.Font
 	TextSize     unit.Sp
 	Background   color.NRGBA
 	CornerRadius unit.Dp

widget/material/checkable.go

@@ -6,6 +6,7 @@ import (
 	"image"
 	"image/color"
 
+	"gioui.org/font"
 	"gioui.org/internal/f32color"
 	"gioui.org/layout"
 	"gioui.org/op"
@@ -19,7 +20,7 @@ import (
 type checkable struct {
 	Label              string
 	Color              color.NRGBA
-	Font               text.Font
+	Font               font.Font
 	TextSize           unit.Sp
 	IconColor          color.NRGBA
 	Size               unit.Dp

widget/material/editor.go

@@ -5,6 +5,7 @@ package material
 import (
 	"image/color"
 
+	"gioui.org/font"
 	"gioui.org/internal/f32color"
 	"gioui.org/layout"
 	"gioui.org/op"
@@ -15,7 +16,7 @@ import (
 )
 
 type EditorStyle struct {
-	Font     text.Font
+	Font     font.Font
 	TextSize unit.Sp
 	// Color is the text color.
 	Color color.NRGBA

widget/material/label.go

@@ -5,6 +5,7 @@ package material
 import (
 	"image/color"
 
+	"gioui.org/font"
 	"gioui.org/internal/f32color"
 	"gioui.org/layout"
 	"gioui.org/op"
@@ -19,7 +20,7 @@ import (
 // the label will be non-interactive.
 type LabelStyle struct {
 	// Face defines the text style.
-	Font text.Font
+	Font font.Font
 	// Color is the text color.
 	Color color.NRGBA
 	// SelectionColor is the color of the background for selected text.
@@ -47,13 +48,13 @@ type LabelStyle struct {
 
 func H1(th *Theme, txt string) LabelStyle {
 	label := Label(th, th.TextSize*96.0/16.0, txt)
-	label.Font.Weight = text.Light
+	label.Font.Weight = font.Light
 	return label
 }
 
 func H2(th *Theme, txt string) LabelStyle {
 	label := Label(th, th.TextSize*60.0/16.0, txt)
-	label.Font.Weight = text.Light
+	label.Font.Weight = font.Light
 	return label
 }
 
@@ -71,7 +72,7 @@ func H5(th *Theme, txt string) LabelStyle {
 
 func H6(th *Theme, txt string) LabelStyle {
 	label := Label(th, th.TextSize*20.0/16.0, txt)
-	label.Font.Weight = text.Medium
+	label.Font.Weight = font.Medium
 	return label
 }
 
@@ -81,7 +82,7 @@ func Subtitle1(th *Theme, txt string) LabelStyle {
 
 func Subtitle2(th *Theme, txt string) LabelStyle {
 	label := Label(th, th.TextSize*14.0/16.0, txt)
-	label.Font.Weight = text.Medium
+	label.Font.Weight = font.Medium
 	return label
 }
 

widget/material/theme.go

@@ -7,6 +7,7 @@ import (
 
 	"golang.org/x/exp/shiny/materialdesign/icons"
 
+	"gioui.org/font"
 	"gioui.org/text"
 	"gioui.org/unit"
 	"gioui.org/widget"
@@ -46,7 +47,7 @@ type Theme struct {
 	FingerSize unit.Dp
 }
 
-func NewTheme(fontCollection []text.FontFace) *Theme {
+func NewTheme(fontCollection []font.FontFace) *Theme {
 	t := &Theme{
 		Shaper: text.NewShaper(fontCollection),
 	}