lua: add a minor helper function

DOCS/man/en/lua.rst

@@ -354,6 +354,10 @@ The ``mp`` module is preloaded, although it can be loaded manually with
     Undo a previous registration with ``mp.register_script_message``. Does
     nothing if the ``name`` wasn't registered.
 
+``mp.osd_message(text [,duration])``
+    Show an OSD message on the screen. ``duration`` is in seconds, and is
+    optional (uses ``--osd-duration`` by default).
+
 mp.msg functions
 ----------------
 

player/lua/defaults.lua

@@ -1,3 +1,5 @@
+
+-- these are used internally by lua.c
 mp.UNKNOWN_TYPE.info = "this value is inserted if the C type is not supported"
 mp.UNKNOWN_TYPE.type = "UNKNOWN_TYPE"
 
@@ -360,4 +362,15 @@ _G.mp_event_loop = function()
     end
 end
 
+-- additional helpers
+
+function mp.osd_message(text, duration)
+    if not duration then
+        duration = "-1"
+    else
+        duration = tostring(math.floor(duration * 1000))
+    end
+    mp.commandv("show_text", text, duration)
+end
+
 return {}