libobs: Fix mouse button hotkeys on linux

The screen index returned from XDefaultScreen is 0-based, and we were
decrementing it before the check to see if it had reached 0 rather than
after, so in the default_screen function it would always end up getting
either the wrong screen or no screen.

When xcb_query_pointer and xcb_query_pointer_reply was called with no
valid screen, it would fail with an error, thus making it so that the
mouse buttons could not be properly captured as hotkeys.

libobs/obs-nix.c

@@ -682,7 +682,7 @@ static xcb_screen_t *default_screen(obs_hotkeys_platform_t *context,
 
 	iter = xcb_setup_roots_iterator(xcb_get_setup(connection));
 	while (iter.rem) {
-		if (--def_screen_idx == 0) {
+		if (def_screen_idx-- == 0) {
 			return iter.data;
 			break;
 		}