[mathlib] Add Blend macro

The Blend macro supports any non-integral type supporting * and +
(float, double, vec4f_t, etc), so it is essentially a scalar VectorBlend
or QuatBlend.

include/QF/mathlib.h

@@ -57,6 +57,8 @@ extern int		nanmask;
 
 #define IS_NAN(x) (((*(int *) (char *) &x) & nanmask) == nanmask)
 
+#define Blend(a,b,blend) ((1 - (blend)) * (a) + (blend) * (b))
+
 #include "QF/math/vector.h"
 #include "QF/math/quaternion.h"
 #include "QF/math/dual.h"