Implement the backpack UI Effect of torchlight

I just wanted to simply borrow torchlight's backpack UI resources, but I found that the backpack UI was opened or closed dynamically. I just want to try to get the whole effect out. after some hard work, we finally achieved it.

 

The entire process mainly includes several technical points:
1. To create a render to texture window with cegui.
This allows us to create a second viewport which can be rendered directly into a cegui widget

2. animation Blending Using one entity with 2 animations from 2 separate skeleton files

3. Create rendertargetlistener for updating UI screen position before render