SBCodepointSequence: Mark buffer as const

[glebm]

The functions never modify the buffer.
Marking it as const avoids the need for const_cast in C++ (as seen in https://github.com/diasurgical/devilutionX/pull/7698/files#diff-34f5922088a0ad010a634e20bd2e1497d6bf759621eec0e03322f99a8022b06e)

The const qualifier was introduced in C89.

[mta452]

I had also considered to mark it as const at one point but I don't remember exactly why I didn't go for it. I think it cased some compatibility issues with C and C++. I'll have to look into it again while making sure that it does not break the contract for existing users. Otherwise, there will be a need to publish a major version of the library which isn't justified for this single change only.

[glebm]

I'll have to look into it again while making sure that it does not break the contract for existing users

A non-const can used everywhere where const is expected as a field, so this change should be source-compatible and ABI-compatible with all existing code. I'm guessing you have some projects that use it in ways not covered by existing tests? Thanks for looking into it and testing.