mobile: Bug fix: mutex is necessary for start and update

mobile's Start and Update can be called from different threads.
Especially, on Android, this behavior is noted explicitly at
https://developer.android.com/reference/android/opengl/GLSurfaceView.Renderer

mobile/impl_mobile.go

@@ -18,17 +18,22 @@ package mobile
 
 import (
 	"errors"
+	"sync"
 
 	"github.com/hajimehoshi/ebiten"
 	"github.com/hajimehoshi/ebiten/internal/ui"
 )
 
 var (
-	chError <-chan error
-	running bool
+	chError     <-chan error
+	running     bool
+	mobileMutex sync.Mutex
 )
 
 func update() error {
+	mobileMutex.Lock()
+	defer mobileMutex.Unlock()
+
 	if chError == nil {
 		return errors.New("mobile: chError must not be nil: Start is not called yet?")
 	}
@@ -39,6 +44,9 @@ func update() error {
 }
 
 func start(f func(*ebiten.Image) error, width, height int, scale float64, title string) {
+	mobileMutex.Lock()
+	defer mobileMutex.Unlock()
+
 	running = true
 	chError = ebiten.RunWithoutMainLoop(f, width, height, scale, title)
 }