Fix window resize not always working

Change doresize to atomic_flag to prevent race condition

src/win/win.c

@@ -35,6 +35,7 @@
 #include <direct.h>
 #include <wchar.h>
 #include <io.h>
+#include <stdatomic.h>
 #define HAVE_STDARG_H
 #include <86box/86box.h>
 #include <86box/config.h>
@@ -550,12 +551,11 @@ main_thread(void *param)
 		// Sleep(1);
 
 	/* If needed, handle a screen resize. */
-	if (doresize && !video_fullscreen && !is_quit) {
+	if (!atomic_flag_test_and_set(&doresize) && !video_fullscreen && !is_quit) {
 		if (vid_resize & 2)
 			plat_resize(fixed_size_x, fixed_size_y);
 		else
 			plat_resize(scrnsz_x, scrnsz_y);
-		doresize = 0;
 	}
     }
 
@@ -1162,7 +1162,7 @@ plat_setfullscreen(int on)
     video_fullscreen &= 1;
     video_force_resize_set(1);
     if (!(on & 1))
-	doresize = 1;
+        atomic_flag_clear(&doresize);
 
     win_mouse_init();