#2678 transform_xy rejects argument constructed with make_transform

[Robert Dodier]

I think make_transform is suppose to construct a function suitable for transform_xy. Here's what I get:

(%i13) plot3d ([r, theta, sqrt (sin (theta))], [r, 0, 1], [theta, 0, 2*%pi],
  [transform_xy, make_transform ([r, theta, z], r*cos(theta), r*sin(theta), z)])$
Wrong argument for plot option "transform_xy". Expecting a symbol
            , true or false but found "#<COMPILED-FUNCTION $MAKE_TRANSFORM-1>".

[Jaime E. Villate]

Fixed by commit [1291fe], with the patch contributed by Tomio ((有坂 富雄). The correct command to show the use transform_xy would be something such as:

plot3d (sqrt (abs(sin (theta))), [r, 0, 1], [theta, 0, 2*%pi],
[transform_xy, make_transform ([r, theta, z], r*cos(theta), r*sin(theta), z)]);