Windows: don't beep on Alt-* key combos (Thanks, historic_bruno!).

Fixes Bugzilla 2669.

src/video/windows/SDL_windowsevents.c

@@ -791,9 +791,13 @@ WIN_WindowProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
         }
         return (1);
 
-#if defined(SC_SCREENSAVE) || defined(SC_MONITORPOWER)
     case WM_SYSCOMMAND:
         {
+			if ((wParam & 0xFFF0) == SC_KEYMENU) {
+				return (0);
+			}
+
+#if defined(SC_SCREENSAVE) || defined(SC_MONITORPOWER)
             /* Don't start the screensaver or blank the monitor in fullscreen apps */
             if ((wParam & 0xFFF0) == SC_SCREENSAVE ||
                 (wParam & 0xFFF0) == SC_MONITORPOWER) {
@@ -801,9 +805,9 @@ WIN_WindowProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
                     return (0);
                 }
             }
+#endif /* System has screensaver support */
         }
         break;
-#endif /* System has screensaver support */
 
     case WM_CLOSE:
         {