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.go

@@ -26,6 +26,7 @@ type Window struct {
 
 type context interface {
 	Functions() *glimpl.Functions
+	Backend() (gpu.Backend, error)
 	MakeCurrent() error
 	ReleaseCurrent()
 	Release()
@@ -53,7 +54,7 @@ func NewWindow(width, height int) (*Window, error) {
 			ctx.Release()
 			return err
 		}
-		backend, err := gl.NewBackend(f)
+		backend, err := ctx.Backend()
 		if err != nil {
 			fbo.Release()
 			ctx.Release()