Refactor rendering systems to use let-else

[nicopap]

Objective

Some rendering system did heavy use of if let, and could be improved by using let else.

Solution

• Reduce rightward drift by using let-else over if-let
• Extract value-to-key mappings to their own functions so that the system is less bloated, easier to understand
• Use a let binding instead of untupling in closure argument to reduce indentation

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[nicopap]

Wait, there is also wireframe.rs that can get the same change.

[nicopap]

There is still more in render/light.rs

[james7132]

Not a huge fan of how rustfmt handles let-else, but less heavy indentation in these huge functions is definitely the lesser evil here.

[IceSentry]

I personally preferred having all the pipeline_key stuff inlined instead of in separate functions but other than that I like using let else everywhere so I'll just approve this.