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/gl_macos.go

@@ -6,6 +6,8 @@ package window
 
 import (
 	"gioui.org/app/internal/glimpl"
+	"gioui.org/gpu"
+	"gioui.org/gpu/gl"
 )
 
 /*
@@ -40,8 +42,8 @@ func newContext(w *window) (*context, error) {
 	return c, nil
 }
 
-func (c *context) Functions() *glimpl.Functions {
-	return c.c
+func (c *context) Backend() (gpu.Backend, error) {
+	return gl.NewBackend(c.c)
 }
 
 func (c *context) Release() {