Fix a rare crash when running simulate on macOS.

The bug was introduced in 2f0fb1e4ef. The glfwSwapBuffers call should occur on the main UI thread, not the CoreVideo DisplayLink thread.

PiperOrigin-RevId: 517172202
Change-Id: I2491f2194765019748198cd0cc61ca9263331b01

simulate/glfw_corevideo.h

@@ -19,6 +19,7 @@
 #error "This header only works on macOS."
 #endif
 
+#include <atomic>
 #include <condition_variable>
 #include <mutex>
 
@@ -37,8 +38,8 @@ class GlfwCoreVideo {
  public:
   GlfwCoreVideo(GLFWwindow* window);
   ~GlfwCoreVideo();
-  void EnqueueSwap();
-  void WaitForSwap();
+
+  void WaitForDisplayRefresh();
   int DisplayLinkCallback();
   void UpdateDisplayLink();
 
@@ -46,7 +47,7 @@ class GlfwCoreVideo {
   GLFWwindow* window_;
   CVDisplayLinkRef display_link_;
 
-  bool second_buffer_has_content_;
+  std::atomic_bool waiting_;
   std::mutex mu_;
   std::condition_variable cond_;
 };