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>

app/headless/backend_test.go

@@ -170,7 +170,7 @@ func newBackend(t *testing.T) backend.Device {
 	if err := ctx.MakeCurrent(); err != nil {
 		t.Fatal(err)
 	}
-	b, err := ctx.Backend()
+	b, err := backend.NewDevice(ctx.API())
 	if err != nil {
 		t.Fatal(err)
 	}