Fix window resize not always working

Change doresize to atomic_flag to prevent race condition

src/unix/unix.c

@@ -18,6 +18,7 @@
 #include <inttypes.h>
 #include <dlfcn.h>
 #include <wchar.h>
+#include <stdatomic.h>
 
 #include <86box/86box.h>
 #include <86box/keyboard.h>
@@ -531,12 +532,11 @@ main_thread(void *param)
 		SDL_Delay(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;
 	}
     }