add input.get_mouse_delta()

doc/en/scripting/builtins/libinput.md

@@ -48,6 +48,12 @@ input.get_mouse_pos() --> {int, int}
 
 Returns cursor screen position.
 
+```lua
+input.get_mouse_delta() --> {int, int}
+```
+
+Returns cursor movement delta.
+
 ```lua
 input.get_bindings() --> strings array
 ```