console.lua: limit the length of selectable items

Giving very long lines to libass freezes mpv and makes the CPU spin
(though it's fine with terminal output). This is often the case
user-data/mpv/ytdl/json-subprocess-result in the property list. So limit
the length of selectable items to 300 characters (not Unicode aware).
This is enough to fill a 1920x1080 window with font_size=16 and Japanese
text.

player/lua/console.lua

@@ -1814,7 +1814,7 @@ mp.register_script_message('get-input', function (script_name, args)
     if args.items then
         selectable_items = {}
         for i, item in ipairs(args.items) do
-            selectable_items[i] = item:gsub("[\r\n].*", "⋯")
+            selectable_items[i] = item:gsub("[\r\n].*", "⋯"):sub(1, 300)
         end
 
         matches = {}