[Core] [Mono] [GDNative] Rename "linear_interpolate" methods to "lerp"

[aaronfranke]

Lerp is a much better name, it's shorter, more familiar, and it's consistent with lerp, inverse_lerp, and slerp in math_funcs.h, and with Mono's Mathf.Lerp and Slerp methods.

I made the changes for C#, C++, GDScript, and GDNative, but I kept the old "linear_interpolate" methods as well to maintain backwards compatibility.

[akien-mga]

I don't think it a good idea for C# to diverge from the C++, GDNative and GDScript APIs, unless there's a strong reason why it should be different in C# from the other languages (like snake_case to PascalCase, etc.).

I don't think this is the case, it's just a cosmetic change which breaks the correspondence between C++/GDNative/GDScript and C#. If we want to make this change, it should be done for all languages at once next time we decide to break API compatibility for cosmetic changes (#16863).

[aaronfranke]

If desired, I could add lerp without removing linear_interpolate just by copying the code, and we could remove linear_interpolate in 4.0?

[karroffel]

Thanks for the GDNative implementation! Unfortunately, breaking compatibility of the core struct is a NO-GO, so I will have to create a new GDNative core extension for any new changes.

[aaronfranke]

@karroffel I kept the existing methods, what else can be done to preserve compatibility?

As a quick refernece, there's one commit at the bottom which has all the GDNative changes.

If desired I could undo that commit so we can merge the rest. GDNative calling "linear_interpolate" from Core will still work since I left wrapper methods.

[neikeq]

The C# changes look good to me.

[aaronfranke]

I'm adding the breaks compat label since this does break C# compatibility, which wasn't important back when this PR was created, but I think we care about C# compat now.

For GDScript/etc there are still wrapper methods for linear_interpolate so compat isn't broken there but these are easy to remove if desired. Otherwise this should be good to merge.

There aren't deprecated aliases or anything as @akien-mga mentions above, but the wrapper methods can easily be converted when/if that system is added.

[neikeq]

C# compat matters if this is going to be cherry-picked. For 4.0 I plan to do a lot of breaking changes though, if I have the time.

[akien-mga]

You can fully break compatibility, the aim is to make the API more consistent in 4.0 so we don't need to drag around compat methods forever.

[aaronfranke]

@akien-mga Done.

[akien-mga]

Will need yet another rebase. We can now break compat for GDNative too in 4.0, we'll want the API to match Core anyway so there's no way to presence compatibility without tons of outdated bindings.

[akien-mga]

Thanks!