mobile/ebitenmobileview: Allow to call update before start

This fixes an issue that Update on another thread returns error
when SetGame is not called. As SetGame is permitted to be called
later, Update should not return error.

mobile/ebitenmobileview/impl_mobile.go

@@ -22,23 +22,19 @@ package ebitenmobileview
 import "C"
 
 import (
-	"errors"
-
 	"github.com/hajimehoshi/ebiten"
 	"github.com/hajimehoshi/ebiten/internal/uidriver/mobile"
 )
 
 var (
 	chError <-chan error
-	running bool
 )
 
 func update() error {
 	if chError == nil {
-		return errors.New("ebitenmobileview: chError must not be nil: Start is not called yet?")
-	}
-	if !running {
-		return errors.New("ebitenmobileview: start must be called ahead of update")
+		// start is not called yet, but as update can be called from another thread, it is OK. Just ignore
+		// this.
+		return nil
 	}
 
 	select {
@@ -52,7 +48,6 @@ func update() error {
 }
 
 func start(f func(*ebiten.Image) error, width, height int, scale float64) {
-	running = true
 	// The last argument 'title' is not used on mobile platforms, so just pass an empty string.
 	chError = ebiten.RunWithoutMainLoop(f, width, height, scale, "")
 }