font: add a central font registry

Package font holds a singleton text.Shaper for general use. Subpackages
call Register to add fonts to the registry.

The intention is that programs can add a typeface by importing a package:

import _ "gioui.org/font/gofont"

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

font/font.go

@@ -0,0 +1,36 @@
+// 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.Shaper)
+)
+
+// Default returns a singleton *text.Shaper that contains
+// the registered fonts.
+func Default() *text.Shaper {
+	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)
+}