Use stdlib for trigonometric functions

Profiling showed this performed significantly faster than the RageFast_ implementations.

src/Tween.cpp

@@ -33,7 +33,7 @@ struct TweenDecelerate: public ITween
 };
 struct TweenSpring: public ITween
 {
-	float Tween( float f ) const { return 1 - RageFastCos( f*PI*2.5f )/(1+f*3); }
+	float Tween( float f ) const { return 1 - std::cos( f*PI*2.5f )/(1+f*3); }
 	ITween *Copy() const { return new TweenSpring(*this); }
 };