Implement SSGIEffect

### Description

Implement a `SSGIEffect`.

References:
- https://github.com/0beqz/realism-effects/tree/main/src/ssgi
- https://arxiv.org/pdf/2301.11376.pdf

### Tasks

- [ ] Create `SSGIEffect`.
- [ ] Create `ssgi` demo.
- [ ] Add unit test.

### Implementation Details

- Use G-Buffer components (depth, normals, ORM).