Frame Graph Implementation

Here are a number of things that are still to be implemented to work with frame graphs.
This list is not ranked in order of importance; some items are more important than others, and some will likely be postponed until a request is made, as they are not a priority:

* [ ] geometry renderer task:
* [ ] adds support for splat meshes
* [ ] improves the "beauty" texture (outputTexture) (adds support for sprites, particles, bounding boxes, outline renderer)
* [ ] handles `scene.needsPreviousWorldMatrices` correctly (should be true if any renderer generates the velocity / linear velocity texture)
* [ ] allows custom geometry textures
* [ ] min/max reducer (it works with frame graphs, but we could improve it by reusing the geometry depth texture if already generated in the graph)
* [x] IBL shadows
* [ ] Atmosphere add-on
* [ ] procedural textures (they work with frame graphs, but we could integrate them better)
* [ ] RSM gi
* [ ] fluid renderer
* [ ] History texture: being able to ping/pong the texture multiple times in the same frame
* [ ] NME with prepass texture block (https://playground.babylonjs.com/#WW65SN#17)