x11: pass a long to XChangeProperty, not an int.

The Xlib documentation demands that 32-bit values here be passed in a long,
even when long itself isn't a 32-bit value. Otherwise libx11 might read
memory incorrectly.

Fixes Bugzilla #3692.

src/video/x11/SDL_x11window.c

@@ -375,7 +375,7 @@ X11_CreateWindow(_THIS, SDL_Window * window)
     Atom _NET_WM_WINDOW_TYPE;
     Atom wintype;
     const char *wintype_name = NULL;
-    int compositor = 1;
+    long compositor = 1;
     Atom _NET_WM_PID;
     Atom XdndAware, xdnd_version = 5;
     long fevent = 0;