lua: make later key bindings always have higher priority

Later calls to mp.add_key_binding() should take priority over previous
calls with the same key. Until now, the order was random (due to using
table pairs() iteration order).

Do this by simply sorting by a counter that is never reset. Since
input.c also gives later bindings priority, this works out.

Calling mp.remove_key_binding() on a newer binding makes an older still
existing binding with the same key active again. New bindings override
older ones, but do not overwrite them. I think these are good semantics
for most use cases.

(Note that the Lua code cannot determine whether two bindings use the
same key. Keys are strings, and two different strings could refer to the
same key. The code does not have access to input.c's key name
normalization, so it cannot compare them.)

player/lua/defaults.lua

@@ -131,6 +131,7 @@ end
 -- "Newer" and more convenient API
 
 local key_bindings = {}
+local key_binding_counter = 0
 
 local function update_key_bindings()
     for i = 1, 2 do
@@ -143,12 +144,19 @@ local function update_key_bindings()
             section = "input_forced_" .. mp.script_name
             flags = "force"
         end
-        local cfg = ""
+        local bindings = {}
         for k, v in pairs(key_bindings) do
             if v.bind and v.forced ~= def then
-                cfg = cfg .. v.bind .. "\n"
+                bindings[#bindings + 1] = v
             end
         end
+        table.sort(bindings, function(a, b)
+            return a.priority < b.priority
+        end)
+        local cfg = ""
+        for _, v in ipairs(bindings) do
+            cfg = cfg .. v.bind .. "\n"
+        end
         mp.input_define_section(section, cfg, flags)
         -- TODO: remove the section if the script is stopped
         mp.input_enable_section(section, "allow-hide-cursor+allow-vo-dragging")
@@ -217,6 +225,9 @@ local function add_binding(attrs, key, name, fn, rp)
         attrs.bind = key .. " script-binding " .. mp.script_name .. "/" .. name
     end
     attrs.name = name
+    -- new bindings override old ones (but do not overwrite them)
+    key_binding_counter = key_binding_counter + 1
+    attrs.priority = key_binding_counter
     key_bindings[name] = attrs
     update_key_bindings()
     dispatch_key_bindings[name] = key_cb