options: --wid can be set at any time

After being bitten by this, I decided that this mostly unnecessary
requirement sucks.

Allowing this makes it easier to use libmpv, because it can be set after
mpv_initialize(). The latest reasonable time an API user can set this
variable is before actually loading a file.

The previous 2 commits make sure nothing bad can happen if the option is
changed at runtime even if a VO is active. The Cocoa backend should be
fine and doesn't need a change.

options/options.c

@@ -441,7 +441,7 @@ const m_option_t mp_opts[] = {
     OPT_FLAG("cursor-autohide-fs-only", cursor_autohide_fs, 0),
     OPT_FLAG("stop-screensaver", stop_screensaver, 0),
 
-    OPT_INT64("wid", vo.WinID, CONF_GLOBAL),
+    OPT_INT64("wid", vo.WinID, 0),
 #if HAVE_X11
     OPT_CHOICE("x11-netwm", vo.x11_netwm, 0,
                ({"auto", 0}, {"no", -1}, {"yes", 1})),