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>

text/gotext.go

@@ -18,6 +18,7 @@ import (
 	"golang.org/x/text/unicode/bidi"
 
 	"gioui.org/f32"
+	giofont "gioui.org/font"
 	"gioui.org/io/system"
 	"gioui.org/op"
 	"gioui.org/op/clip"
@@ -150,24 +151,24 @@ type runLayout struct {
 
 // faceOrderer chooses the order in which faces should be applied to text.
 type faceOrderer struct {
-	def                 Font
+	def                 giofont.Font
 	faceScratch         []font.Face
-	fontDefaultOrder    map[Font]int
-	defaultOrderedFonts []Font
-	faces               map[Font]font.Face
+	fontDefaultOrder    map[giofont.Font]int
+	defaultOrderedFonts []giofont.Font
+	faces               map[giofont.Font]font.Face
 	faceToIndex         map[font.Face]int
-	fonts               []Font
+	fonts               []giofont.Font
 }
 
-func (f *faceOrderer) insert(fnt Font, face font.Face) {
+func (f *faceOrderer) insert(fnt giofont.Font, face font.Face) {
 	if len(f.fonts) == 0 {
 		f.def = fnt
 	}
 	if f.fontDefaultOrder == nil {
-		f.fontDefaultOrder = make(map[Font]int)
+		f.fontDefaultOrder = make(map[giofont.Font]int)
 	}
 	if f.faces == nil {
-		f.faces = make(map[Font]font.Face)
+		f.faces = make(map[giofont.Font]font.Face)
 		f.faceToIndex = make(map[font.Face]int)
 	}
 	f.fontDefaultOrder[fnt] = len(f.faceScratch)
@@ -198,7 +199,7 @@ func (c *faceOrderer) faceFor(idx int) font.Face {
 // TODO(whereswaldon): this function could sort all faces by appropriateness for the
 // given font characteristics. This would ensure that (if possible) text using a
 // fallback font would select similar weights and emphases to the primary font.
-func (c *faceOrderer) sortedFacesForStyle(font Font) []font.Face {
+func (c *faceOrderer) sortedFacesForStyle(font giofont.Font) []font.Face {
 	c.resetFontOrder()
 	primary, ok := c.fontForStyle(font)
 	if !ok {
@@ -213,11 +214,11 @@ func (c *faceOrderer) sortedFacesForStyle(font Font) []font.Face {
 
 // fontForStyle returns the closest existing font to the requested font within the
 // same typeface.
-func (c *faceOrderer) fontForStyle(font Font) (Font, bool) {
+func (c *faceOrderer) fontForStyle(font giofont.Font) (giofont.Font, bool) {
 	if closest, ok := closestFont(font, c.fonts); ok {
 		return closest, true
 	}
-	font.Style = Regular
+	font.Style = giofont.Regular
 	if closest, ok := closestFont(font, c.fonts); ok {
 		return closest, true
 	}
@@ -226,7 +227,7 @@ func (c *faceOrderer) fontForStyle(font Font) (Font, bool) {
 
 // faces returns a slice of faces with primary as the first element and
 // the remaining faces ordered by insertion order.
-func (f *faceOrderer) sorted(primary Font) []font.Face {
+func (f *faceOrderer) sorted(primary giofont.Font) []font.Face {
 	// If we find primary, put it first, and omit it from the below sort.
 	lowest := 0
 	for i := range f.fonts {
@@ -875,9 +876,9 @@ func computeVisualOrder(l *line) {
 
 // closestFont returns the closest Font in available by weight.
 // In case of equality the lighter weight will be returned.
-func closestFont(lookup Font, available []Font) (Font, bool) {
+func closestFont(lookup giofont.Font, available []giofont.Font) (giofont.Font, bool) {
 	found := false
-	var match Font
+	var match giofont.Font
 	for _, cf := range available {
 		if cf == lookup {
 			return lookup, true
@@ -902,7 +903,7 @@ func closestFont(lookup Font, available []Font) (Font, bool) {
 }
 
 // weightDistance returns the distance value between two font weights.
-func weightDistance(wa Weight, wb Weight) int {
+func weightDistance(wa giofont.Weight, wb giofont.Weight) int {
 	// Avoid dealing with negative Weight values.
 	a := int(wa) + 400
 	b := int(wb) + 400