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/loop.go

@@ -67,12 +67,7 @@ func (l *renderLoop) renderLoop(ctx window.Context) error {
 			initErr <- err
 			return
 		}
-		b, err := ctx.Backend()
-		if err != nil {
-			initErr <- err
-			return
-		}
-		g, err := gpu.New(b)
+		g, err := gpu.New(ctx.API())
 		if err != nil {
 			initErr <- err
 			return