text,widget/material: make font collections explicit

Before this change, package font implemented a global font registry,
with the usual problems of package global state.

This change deletes the global registry and introduces the text.Collection
type for representing a list of fonts and their faces. Collection exports
Lookup that finds the closest match and its face.

The existing FontRegistry is renamed to Cache to reflect its new limited
functionality: a cache of shapes and measurements on top of a Collection.

Then, material.NewTheme is changed to take a Collection and initialize
a Cache.

Updates gio#19 because multiple windows require a separate (writable) Cache per
window, while (read-only) Collections may be shared.

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

widget/material/theme.go

@@ -6,7 +6,6 @@ import (
 	"image/color"
 
 	"gioui.org/f32"
-	"gioui.org/font"
 	"gioui.org/layout"
 	"gioui.org/op/paint"
 	"gioui.org/text"
@@ -30,9 +29,9 @@ type Theme struct {
 	radioUncheckedIcon    *widget.Icon
 }
 
-func NewTheme() *Theme {
+func NewTheme(col *text.Collection) *Theme {
 	t := &Theme{
-		Shaper: font.Default(),
+		Shaper: text.NewCache(col),
 	}
 	t.Color.Primary = rgb(0x3f51b5)
 	t.Color.Text = rgb(0x000000)