Draw horizontal lines and labels in the editor performance monitors

[Calinou]

This partially addresses godotengine/godot-proposals#1014.

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[akien-mga]

Why are these constexpr and not just const?

Also, we should likely start a broader discussion on whether this kind of micro optimizations actually make a difference at all. If yes, we should use them consistently in new code. If not, we should probably not refactor existing code to use it.

[Calinou]

I changed them to use const. And yes, we should probably look at using constexpr instead of symbolic #defines in the future.

[akien-mga]

I mean, does using const for local variables really make a performance difference? Or are compilers clever enough to optimize all this code already and us doing it manually is overkill? I think this should be benchmarked before continuing to make all local variables const.

[Calinou]

I think it's more about code readability than performance. I don't think const local variables make any difference for compiler optimization. However, with const, it's guaranteed the variable isn't being reassigned below. This can make the code significantly easier to read and less error-prone. I've seen lots of old code in Godot where variables are being reassigned for no good reason, and that makes it harder to reason about the code.

For this reason, using const should also make online code reviews a bit easier 🙂

The only case where I consider const to be overkill is in function parameters for primitive types (like the ISO C++ Core Guidelines say).

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 3.2.2.