[vec] Add 2D cross product

Add 2D cross product (returns a scalar value).

For consistency with the rest of the API, the 3D cross product has been renamed
to cross3().

vec.lua

@@ -26,8 +26,13 @@ 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)
+--- Vector cross product in 2D, returning a scalar.
+function vec.cross(x1,y1, x2,y2)
+	return x1*y2 - y1*x2
+end
+
+--- Vector cross product in 3D, returning a vector.
+function vec.cross3(x1,y1,z1, x2,y2,z2)
     return y1*z2 - z1*y2,
            z1*x2 - x1*z2,
            x1*y2 - y1*x2