app/internal/window: fix iOS context creation

NewBackend was recently changed to take a context, which must only
be specified on GOOS=js.

Fixes gio#189

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

app/internal/window/gl_ios.go

@@ -57,7 +57,7 @@ func newContext(w *window) (*context, error) {
 }
 
 func (c *context) Backend() (backend.Device, error) {
-	return gl.NewBackend(c.c)
+	return gl.NewBackend(nil)
 }
 
 func (c *context) Release() {