Impossible to compile rive using amalgamated (also called unity) builds

[kunitoki]

I'm delivering the Rive runtime as a single .h + .cpp file to speed up compilation using cmake UNITY_BUILD but i'm encountering several issues:

• The library has the bad practice of using namespace rive in cpp files before declaring rive classes implementation. Consider always wrapping Rive code around namespace rive { ... } or it ends up with ambiguous symbols.
• There is a missing pragma once in "rive/core/reader.h" and it being included in multiple translation units (that end up being the same in a unity build)
• There are duplicated symbols, like rive::lerp that could be isolated in a math utils file.

It would be great if you could allow making the library usable with amalgamated builds.