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>

font/font.go

@@ -1,36 +0,0 @@
-// SPDX-License-Identifier: Unlicense OR MIT
-
-// Package font implements a central font registry.
-package font
-
-import (
-	"sync"
-
-	"gioui.org/text"
-)
-
-var (
-	mu          sync.Mutex
-	initialized bool
-	shaper      = new(text.FontRegistry)
-)
-
-// Default returns a singleton *text.Shaper that contains
-// the registered fonts.
-func Default() *text.FontRegistry {
-	mu.Lock()
-	defer mu.Unlock()
-	initialized = true
-	return shaper
-}
-
-// Register a face. Register panics if Default has been
-// called.
-func Register(font text.Font, face text.Face) {
-	mu.Lock()
-	defer mu.Unlock()
-	if initialized {
-		panic("Register must be called before Default")
-	}
-	shaper.Register(font, face)
-}