mobile: Lock the OS thread before using mutex

As switching the OS thread by mutex could theoretically happen
(I think this is almost 0% in Ebiten's case), locking the OS
thread should happen before the mutex.

mobile/impl_mobile.go

@@ -18,7 +18,6 @@ package mobile
 
 import (
 	"errors"
-	"runtime"
 
 	"github.com/hajimehoshi/ebiten"
 	"github.com/hajimehoshi/ebiten/internal/uidriver/mobile"
@@ -30,9 +29,6 @@ var (
 )
 
 func update() error {
-	runtime.LockOSThread()
-	defer runtime.UnlockOSThread()
-
 	if chError == nil {
 		return errors.New("mobile: chError must not be nil: Start is not called yet?")
 	}

mobile/mobile.go

@@ -21,6 +21,7 @@
 package mobile
 
 import (
+	"runtime"
 	"sync"
 
 	"github.com/hajimehoshi/ebiten"
@@ -63,7 +64,9 @@ func Start(f func(*ebiten.Image) error, width, height int, scale float64, title
 //
 // Update returns error when 1) OpenGL error happens, or 2) f in Start returns error samely as ebiten.Run.
 func Update() error {
-	// TODO: Is it safe to call Lock before locking the OS thread?
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
+
 	mobileMutex.Lock()
 	defer mobileMutex.Unlock()