osc: explicitly re-init the osc on a change in border visibility

I had previously wondered whether to do this, but in my testing
with x11 and wayland, the osc was being re-inited on a border
toggle already so I didn't add it.

However, on win32, things are different and there is no re-init
when toggling borders. I belive this is because the active window
size doesn't change in anyway, while on x11/wayland, toggling the
border actually changes the window size - and that trigger a re-init.

So, let's just be explicit and request a re-init when the border
is toggled.

player/lua/osc.lua

@@ -2497,6 +2497,7 @@ mp.observe_property("fullscreen", "bool",
 mp.observe_property("border", "bool",
     function(name, val)
         state.border = val
+        request_init()
     end
 )
 mp.observe_property("idle-active", "bool",