move MeshT = typeof(mesh) out of at-threaded loops

[ranocha]

This loop-invariant code motion performs slightly better and leads to less allocation with multiple threads, see #3017 (comment).

[github-actions]

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[vchuravy]

This loop-invariant code motion performs slightly better and leads to less allocation with multiple threads,

It's a bit funny, because we are not calculating anything, but we are now passing a ghost argument (the type) instead of the actual mesh struct

[ranocha]

Yes, exactly. This leads to less allocations (at least when using Polyester.jl) since less wrapping is required. Do you approve this PR?

[MarcoArtiano]

Looks good to me, thanks!

ps: I've tested again and we save 5 seconds and 20MiB on the examples/p4est_2d_dgsem/elixir_euler_subsonic_cylinder.jl, with 16 threads vs main (94 seconds vs 98.8 seconds, and 350 MiB vs 370 MiB). The time might be influenced by some noise, but on average is faster.

[ranocha]

Thanks!