use fmt::format while clang doesn't support std::format yet

Index: rts/System/float4.h
--- rts/System/float4.h.orig
+++ rts/System/float4.h
@@ -108,7 +108,7 @@ struct float4 : public float3
 	}
 
 	std::string str() const {
-		return std::format("float4({:.3f}, {:.3f}, {:.3f}, {:.3f})", x, y, z, w);
+		return fmt::format("float4({:.3f}, {:.3f}, {:.3f}, {:.3f})", x, y, z, w);
 	}
 
 
