Fix DialogBrowse crashing when deleting the last item nullptr. In iOS use std::recursive_mutex instead of the custom check.

PanoPainter/GameViewController.m

@@ -39,23 +39,26 @@ int t_count = 0;
 glm::vec2 t_pos;
 int lock_count = 0;
 NSThread* lock_thread;
+std::recursive_mutex lock_mutex;
 
 @implementation GameViewController
 - (void)async_lock
 {
-    if (lock_thread != [NSThread currentThread] || lock_count == 0)
-        [gl_lock lock];
-    lock_thread = [NSThread currentThread];
-    lock_count++;
+    lock_mutex.lock();
+//    if (lock_thread != [NSThread currentThread] || lock_count == 0)
+//        [gl_lock lock];
+//    lock_thread = [NSThread currentThread];
+//    lock_count++;
     [EAGLContext setCurrentContext:self.context];
     GLKView* view = (GLKView*)self.view;
     [view bindDrawable];
 }
 - (void)async_unlock
 {
-    lock_count--;
-    if (lock_count == 0)
-        [gl_lock unlock];
+//    lock_count--;
+//    if (lock_count == 0)
+//        [gl_lock unlock];
+    lock_mutex.unlock();
 }
 - (void)async_swap
 {