gpu: Move Bind methods to Backend

Having Backend.Bind* methods better matches both opengl and d3d.

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

app/headless/headless.go

@@ -59,7 +59,7 @@ func NewWindow(width, height int) (*Window, error) {
 			fboTex.Release()
 			return err
 		}
-		fbo.Bind()
+		backend.BindFramebuffer(fbo)
 		gp, err := gpu.New(backend)
 		if err != nil {
 			fbo.Release()