Fix window resize not always working

Change doresize to atomic_flag to prevent race condition

src/86box.c

@@ -28,6 +28,8 @@
 #include <string.h>
 #include <time.h>
 #include <wchar.h>
+#include <stdatomic.h>
+
 #ifndef _WIN32
 #include <pwd.h>
 #endif
@@ -97,7 +99,7 @@
 /* Stuff that used to be globally declared in plat.h but is now extern there
    and declared here instead. */
 int		dopause;		/* system is paused */
-int		doresize;			/* screen resize requested */
+atomic_flag		doresize;			/* screen resize requested */
 volatile int		is_quit;				/* system exit requested */
 uint64_t	timer_freq;
 char        emu_version[200];		/* version ID string */
@@ -1286,9 +1288,7 @@ set_screen_size(int x, int y)
 
     /* If the resolution has changed, let the main thread handle it. */
     if ((owsx != scrnsz_x) || (owsy != scrnsz_y))
-	doresize = 1;
-    else
-	doresize = 0;
+		atomic_flag_clear(&doresize);
 }