Add basic win32 example

[ZhenFTW]

As the title says. Please add a basic win32 example. Most examples relies on cross platform windows library and none of it gives the best solution to handle events in the direct os way. I'm having trouble setting up wakeup events and render gl context in simple platforms using win32 for example or trying it on android application in jni using opengl context. I'm having trouble setting up mpv_set_wakeup_callback and mpv_render_context_set_update_callback. How do I do it without sdl or other cross platform libraries? I looked into mpv-android but the projects requires mpv_wakeup and pthread to work with events. I'm not sure how would I do that. Hope I get a proper response or sample demo as this looks pretty hard for me and I hope on embedding mpv on flutter. But most examples relies on cross platform toolkit and every os has their own implementation of mpv_set_wakeup_callback and mpv_render_context_set_update.