Interior mutablilty for re_renderer's static resource pools (RenderPipeline/Shader/Layouts/etc.)

[Wumpf]

What

• Prerequisite for Parallelize space views #1325
• Followed by Make draw data creation no longer require a mutable re_renderer context  #4422

The dynamic resource pool (textures, buffers, bindgroups) was already interior mutable. The static resource pool (mostly append only pool) however, still required mutable access. This PR addresses this.

The actual tricky part is the implications this has on the draw step: wgpu::RenderPass<'a> expects all passed in references to live for 'a. This isn't much of a problem we have with resources from the dynamic resource pool since there the handles are actually Arcs to the underlying wgpu resources. However we can't do this with RenderPipelines (the only relevant static resource for this case) though since we want to be able to swap out what the handle points to for shader reloading. This means we need to hold the lock read lock on render pipelines for the entirety of the pass recording.
This is quite nice and easy for our ViewBuilder draws (== individual space views), but hard for the "compositing step" where we draw into egui's render pass. This is solved by taking out all render pipelines of the lock and putting them back in at the start of the frame.
Once wgpu lifts this restrictions we can simplify things a bit!

(to be continued with follow-up PRs!)

Checklist

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Part of series towards more multithreading in the viewer!

• Selection state is now fully double buffered and has interior mutability #4387
• Enable (selected) new cargo clippy lints #4404
• Time control is now behind a RwLock, making recording config access non-mutable everywhere #4389
• You are here ➡️ Interior mutablilty for re_renderer's static resource pools (RenderPipeline/Shader/Layouts/etc.) #4421
• Make draw data creation no longer require a mutable re_renderer context  #4422
• Make viewer contexts's render context reference non-mutable #4430

[emilk]

Awesome!