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/headless/headless_darwin.go

@@ -4,6 +4,8 @@ package headless
 
 import (
 	"gioui.org/app/internal/glimpl"
+	"gioui.org/gpu"
+	"gioui.org/gpu/gl"
 )
 
 /*
@@ -38,6 +40,10 @@ func (c *nsContext) ReleaseCurrent() {
 	C.gio_headless_clearCurrentContext(c.ctx)
 }
 
+func (c *nsContext) Backend() (gpu.Backend, error) {
+	return gl.NewBackend(c.c)
+}
+
 func (c *nsContext) Functions() *glimpl.Functions {
 	return c.c
 }