lua: batch-update key bindings

Lua scripting implements key bindings by defining an input section with
all the bindings in it. Every add_key_binding() call ran a mpv command
to update this section. This caused a lot of spam at debug log levels.

Reduce the spam and more it efficient by batching updates into a single
mpv command when the script becomes inactive. This is pretty simple,
because there's already the concept of idle handlers.

This requires that the script actually goes to sleep, which might not
happen in various extremely bogus corner cases, such as polling the mpv
message queue with active waiting. Just don't do that.

player/lua/defaults.lua

@@ -132,8 +132,14 @@ end
 
 local key_bindings = {}
 local key_binding_counter = 0
+local key_bindings_dirty = false
+
+function mp.flush_keybindings()
+    if not key_bindings_dirty then
+        return
+    end
+    key_bindings_dirty = false
 
-local function update_key_bindings()
     for i = 1, 2 do
         local section, flags
         local def = i == 1
@@ -229,7 +235,7 @@ local function add_binding(attrs, key, name, fn, rp)
     key_binding_counter = key_binding_counter + 1
     attrs.priority = key_binding_counter
     key_bindings[name] = attrs
-    update_key_bindings()
+    key_bindings_dirty = true
     dispatch_key_bindings[name] = key_cb
     mp.register_script_message(name, msg_cb)
 end
@@ -245,7 +251,7 @@ end
 function mp.remove_key_binding(name)
     key_bindings[name] = nil
     dispatch_key_bindings[name] = nil
-    update_key_bindings()
+    key_bindings_dirty = true
     mp.unregister_script_message(name)
 end
 
@@ -517,6 +523,8 @@ function mp.dispatch_events(allow_wait)
     end
 end
 
+mp.register_idle(mp.flush_keybindings)
+
 -- additional helpers
 
 function mp.osd_message(text, duration)