Immersive Portals - Sodium/Iris/DH Compat Fork

## [Unreleased Changes]

### Fixed

- Sodium 0.8.x compat: `OcclusionCuller.Visitor` was promoted to a top-level
  interface (`RenderSectionVisitor`), and `Viewport.isBoxVisible`'s old
  6-float AABB signature was split into `isBoxVisible(int,int,int)` and
  `isBoxVisibleDirect(float,float,float,float)`.
- Portals rendering through occluding geometry (or falling back to
  compatibility mode) with a shaderpack active, caused by the deferred
  framebuffer's depth-stencil format not matching Iris's actual main
  render target format.
- Framebuffer thrashing/stutter caused by resizing the deferred
  framebuffer array every time the lag-adaptive portal layer count
  toggled.
- Severe stutter (frame time spikes to 900ms+) when a portal is visible
  with Distant Horizons installed, caused by DH doing synchronous LOD
  generation for the portal's alternate-world view every frame.