Added constants PI, TAU and E to the shader language

[Soupstraw]

I added some of the more commonly used constants to the shading language.
Issue #20726

Bugsquad edit: Closes #20726. master version of #52315.

[Chaosus]

Yep, checked, seems worked correctly. Thanks for your first contribution to Godot Engine! These constants are really useful.

[clayjohn]

A couple thoughts

1. It may be easier to just define the constants in the .cpp files with actions.rename
2. You shouldn't define the extra constants in shaders that don't use them. For example, cubemap roughness and cubemap down sampler do not use TAU or E. So there is no need to touch them.

[akien-mga]

Note that once this is ready, we'll ask you to squash the commits into one before this is merged. See PR workflow on instructions on how to rebase/squash commits in your branch.

[clayjohn]

Implementation looks fine to me, but I question why we are just throwing a bunch of constants into the shader code without considering need.

For example, where would LN2 be useful? I don't even know what constant it represents. And it appears it is only used once in the entire Godot codebase.

[Soupstraw]

They just happened to be defined in the same file as PI and TAU. I guess they aren't necessary as long as the shader compiler is smart enough to compute ln(2) at compile time.

[clayjohn]

Thanks for the clarification. And yeah, I think it's best to add the requested constants for now and if anyone requests the others we can add them the same way.

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉