lua: add mp.unregister_event() function

Someone requested this... I think.

DOCS/man/en/lua.rst

@@ -235,6 +235,11 @@ The ``mp`` module is preloaded, although it can be loaded manually with
 
     See `Events`_ and `List of events`_ for details.
 
+``mp.unregister_event(fn)``
+    Undo ``mp.register_event(..., fn)``. This removes all event handlers that
+    are equal to the ``fn`` parameter. This uses normal Lua ``==`` comparison,
+    so be careful when dealing with closures.
+
 ``mp.add_timeout(seconds, fn)``
     Call the given function fn when the given number of seconds has elapsed.
     Note that the number of seconds can be fractional. As of now, the timer

player/lua/defaults.lua

@@ -235,6 +235,32 @@ function mp.register_event(name, cb)
     return mp.request_event(name, true)
 end
 
+function mp.unregister_event(cb)
+    for name, sub in pairs(event_handlers) do
+        local found = false
+        for i, e in ipairs(sub) do
+            if e == cb then
+                found = true
+                break
+            end
+        end
+        if found then
+            -- create a new array, just in case this function was called
+            -- from an event handler
+            local new = {}
+            for i = 1, #sub do
+                if sub[i] ~= cb then
+                    new[#new + 1] = sub[i]
+                end
+            end
+            event_handlers[name] = new
+            if #new == 0 then
+                mp.request_event(name, false)
+            end
+        end
+    end
+end
+
 -- default handlers
 mp.register_event("shutdown", function() mp.keep_running = false end)
 mp.register_event("script-input-dispatch", script_dispatch)