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/egl/egl.go

@@ -12,6 +12,8 @@ import (
 
 	"gioui.org/app/internal/glimpl"
 	"gioui.org/app/internal/srgb"
+	"gioui.org/gpu"
+	"gioui.org/gpu/gl"
 )
 
 type Context struct {
@@ -119,6 +121,10 @@ func (c *Context) Functions() *glimpl.Functions {
 	return c.c
 }
 
+func (c *Context) Backend() (gpu.Backend, error) {
+	return gl.NewBackend(c.c)
+}
+
 func (c *Context) ReleaseSurface() {
 	if c.eglSurf == nilEGLSurface {
 		return