Add lerp alpha helpers to transform

Required for https://graphics.rwth-aachen.de:9000/Glow/glow-extras/merge_requests/5

src/glow/common/warn_unused.hh

 #pragma once
 
+#if __cplusplus >= 201703L
+#define GLOW_WARN_UNUSED [[nodiscard]]
+#else
 #ifdef GLOW_COMPILER_MSVC
 #define GLOW_WARN_UNUSED
 #else
 #define GLOW_WARN_UNUSED __attribute__((warn_unused_result))
 #endif
+#endif