Support for arbitrary aspect ratios in output window

I wonder how difficult it would be to have support for arbitrary aspect ratios in the output, because a square output limits student possibilities to create interesting scenes, since most devices are actually not square. Restricting the window to be square forces them to waste a lot of space for their scene in the horizontal direction, especially when compared with similar environments that target the same population.

One of the things I like the most about CodeWorld is the fact that the output uses abstract display units instead of pixels or other device-dependent units. Also, I think 10x10 is a good default choice for a general drawing. However, I don't see why choosing to build a 15x10 scene would be a problem, especially now that there are built-in controls to zoom and pan the output.

The only possible problem could be the `coordinatePlane` facility, which I think should be a function anyway, so that the span and resolution are configurable. This could be also important for using CodeWorld as a programmable graphing calculator, for which it has a lot of potential.