[vec] Add cross product between vectors.

vec.lua

@@ -25,6 +25,13 @@ function vec.dot3(x1,y1,z1, x2,y2,z2)
     return x1*x2 + y1*y2 + z1*z2
 end
 
+--- Vector cross product.
+function vec.cross(x1,y1,z1, x2,y2,z2)
+    return y1*z2 - z1*y2,
+           z1*x2 - x1*z2,
+           x1*y2 - y1*x2
+end
+
 --- Vector squared length.
 function vec.sqrlen(x,y)
     return x*x + y*y  -- vec.dot(x,y, x,y)