Smooth CLS animation

[hornc]

Make animations smoother by changing the interpreter's handling of the -hold parameter.

Some kind of double buffering which minimises the amount of blank screen time that might arise if the screen is cleared while the next 'frame' is still being assembled.

Can't assume every program is an animation, nor even that every program with a CLS is an animation.

The -hold parameter could be replaced by an -fps, if that seems appropriate.