gpu,gpu/backend: implement generic backend.NewDevice

NewDevice creates a Device given an API, which is the necessary GPU
resources for a backend.

Convert gpu.New to take an API instead of a backend.Device directly.

In turn, this frees us to later unexport the backend package along with
the backend implementations (for now just gioui.org/gpu/gl for OpenGL).
It also allows programs that embed Gio (such as gioui.org/example/glfw)
to freely choose a backend, not just OpenGL.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

gpu/gl/backend.go

@@ -123,12 +123,12 @@ const (
 	storageBindings = 32
 )
 
-// NewBackend returns a new Backend.
-//
-// Pass a WebGL context if GOOS is "js", otherwise pass nil for the current
-// context.
-func NewBackend(ctx Context) (*Backend, error) {
-	f, err := glimpl.NewFunctions(ctx)
+func init() {
+	backend.NewOpenGLDevice = newOpenGLDevice
+}
+
+func newOpenGLDevice(api backend.OpenGL) (backend.Device, error) {
+	f, err := glimpl.NewFunctions(api.Context)
 	if err != nil {
 		return nil, err
 	}