cmd/ebitenmobile: Ensure that rendering never happens on background

cmd/ebitenmobile/gobind.go

@@ -186,16 +186,18 @@ const objcM = `// Code generated by ebitenmobile. DO NOT EDIT.
 }
 
 - (void)glkView:(GLKView*)view drawInRect:(CGRect)rect {
-  if (error_) {
-    return;
-  }
-  NSError* err = nil;
-  EbitenmobileviewUpdate(&err);
-  if (err != nil) {
-    [self performSelectorOnMainThread:@selector(onErrorOnGameUpdate:)
-                           withObject:err
-                        waitUntilDone:NO];
-    error_ = true;
+  @synchronized(self) {
+    if (error_) {
+      return;
+    }
+    NSError* err = nil;
+    EbitenmobileviewUpdate(&err);
+    if (err != nil) {
+      [self performSelectorOnMainThread:@selector(onErrorOnGameUpdate:)
+                             withObject:err
+                          waitUntilDone:NO];
+      error_ = true;
+    }
   }
 }