crash on windows

[ZenYuan]

A crash occurred when the window was minimized after being correctly created. Preliminary analysis: Normally, minimizing the window will set _windowMapped to false, and the window->visible() check will not cause a Swapchain to be recreated upon minimizing. However, in the vsgwin32 example, the Win32 event is processed in the main thread, and the rendering loop runs in a separate thread. It may be that _windowMapped is changed at an inappropriate time, causing the crash.

version：vsg1.1.11；vsgExample1.1.9