app,app/headless: expose Backends from window and headless contexts

This is a refactoring change to prepare for another gpu.Backend
implementations.

Notably, app/loop.go no longer imports gpu/gl.

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

app/internal/window/window.go

@@ -9,7 +9,7 @@ import (
 	"math"
 	"time"
 
-	"gioui.org/app/internal/glimpl"
+	"gioui.org/gpu"
 	"gioui.org/io/event"
 	"gioui.org/io/system"
 	"gioui.org/unit"
@@ -32,7 +32,7 @@ type Callbacks interface {
 }
 
 type Context interface {
-	Functions() *glimpl.Functions
+	Backend() (gpu.Backend, error)
 	Present() error
 	MakeCurrent() error
 	Release()