Undefined method 'z' for Window

[TzA-0x000000001]

Hello.

I wanted to use Ashton with this OpenGL wrapper so I changed the code in order to make it work. And well, it works pretty well if I fix this error: <ASHTON_PATH>/lib/ashton/shader.rb:109:in `enable': undefined method `z' for #<TestWindow:0x305d168 @__swigtype__="_p_Gosu__Window"> (NoMethodError).

So, I just tried adding some lines to the Window class:

def z(&block) ; yield ; end

And it worked! Well... only for some things. I can post_process(){ ... } but I can't add a shader to a specific image and the post processed blocks are ignoring z depth.
I looked for some def z ; ... ; end in the code but found nothing ; so what can I do ? What's that method and how can I rewrite it ?

Thank you for your attention.

Best regards,
TzA