Reinstate our lerp() function, avoid "math.h" header.

For reasons that remain unclear, MSVC likes our lerp() function better than
its own std::lerp() function: publishing the latter into the global namespace,
instead of defining our own, produces fatal argument conversion warnings.

"math.h" publishes all of <cmath> into the global namespace, which causes a
GCC conflict between std::lerp() and our lerp() function. Including <cmath>
instead leaves std::lerp() in the std namespace, eliminating the conflict.

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