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_ios.go

@@ -17,6 +17,7 @@ import (
 	"fmt"
 
 	"gioui.org/app/internal/glimpl"
+	"gioui.org/gpu"
 	"gioui.org/gpu/gl"
 )
 
@@ -50,8 +51,8 @@ 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() {

app/internal/window/gl_js.go

@@ -8,6 +8,8 @@ import (
 
 	"gioui.org/app/internal/glimpl"
 	"gioui.org/app/internal/srgb"
+	"gioui.org/gpu"
+	"gioui.org/gpu/gl"
 )
 
 type context struct {
@@ -45,8 +47,8 @@ func newContext(w *window) (*context, error) {
 	return c, nil
 }
 
-func (c *context) Functions() *glimpl.Functions {
-	return c.f
+func (c *context) Backend() (gpu.Backend, error) {
+	return gl.NewBackend(c.f)
 }
 
 func (c *context) Release() {

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() {

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()