itch.io

It actually works fine with some minor tweaks. 

1. First create a second TextureRect node.
2. change its texture to "CanvasTexture".
3. under "Ordering" increase it's Z index to 1.
4. change it's material to your shader and drop the shader code onto it. 
5. Then near the top of the shader code add this line:
   1. uniform sampler2D SCREEN_TEXTURE : hint_screen_texture, filter_linear_mipmap;

and viola: